Praise the sun. Our Dark Souls Walkthrough provides tips for surviving the adventure, a guide to the entire world, and hints for slaying bosses.
When you arrive in the Undead Parish, it likely will come as a bit of a relief. However, you're not safe yet. You still haven't found a bonfire, and you won't yet for awhile.
From the entrance to the parish, start forward. There will be an armored soldier or two waiting in plain sight. Take out any of those characters and start into the wider arena ahead. You'll see a giant boar situated on the distant steps. It is surrounded by enemies that you don't want to have to face. You can lure them toward you one at a time by pelting them with arrows and then letting them charge you.
Once you have dealt with those soldiers, start forward through the high archway. Look quickly to your right, as another armored soldier likes to hide in the corner. Deal with him carefully; he has stone columns that make it difficult to approach him, but he's good at spearing you from his point of relative safety. Once you take care of him, head left along the wall where he was positioned and you'll find some steps leading up to a higher floor.
As you arrive on that higher floor, there's a soldier with a shield and lance ahead of you. Beyond him, there also are two archers. You should rush around the armored soldier and take out the archers first, so that they don't pick you apart. Then head back around the armored soldier and look nearby to find another standard soldier who is hiding in the area. Deal with him and then finally the first armored soldier that you found. Then you're free to loot bodies. At the end of the archway where the archers were positioned, you'll find the Alluring Skull lying on a corpse.
Now you're ready to deal with the boar. He's still positioned below. If you made a lot of noise and caught his attention, he may be pacing around the area. Stand near the drop-off area and toss an alluring skull on the ground. The boar should come to investigate and will be standing roughly below you. Quickly drop down from the ledge and use your weapon as you do so to land a fierce blow. With any luck, that will instantly kill the boar. If it doesn't, you'll have to dance around him as he charges and then hit him with your melee weapon to slowly drain his life meter. He's not especially difficult, actually.
When the boar is toast, you can freely investigate the surrounding area. Near the burning corpse, there's a set of steps that descends toward a new chamber. Start down those steps and as you round the corner you'll see an enemy waiting in the hallway. He's likely to run upon spotting you, but don't follow. He is trying to lead you into a trap. Instead, slowly approach with your shield raised. You should see a ledge ahead of you in the room. There's another enemy waiting there. You can hit him with an arrow to draw him toward you. Then, quickly prepare your shield and tend to him as he gets near. The ruckus may draw two more enemies toward you who are waiting in the next room. Deal with them in the usual manner, then head into the room and climb to the level where the one enemy was standing near the tables.
As you climb to that level, note that there's probably a guy still waiting to the right in some barrels, and another may rush you from the left. Work quickly to take care of both of them, then head past the tables and descend to a dead-end corridor where another goon is waiting. Take him out, then investigate the body just beyond him to obtain the Mystery Key. Now return to the upper level with the tables and look along the back wall. You should see a red ladder. You can climb it to an overhead chamber.
At the top of those stairs, two weak enemies are waiting. Take them out, then look to the left. Ceiling beams lead in that direction and you'll spy a corpse with a glowing item. If you head out onto the beam, you can retrieve a Large Soul of a Lost Undead from the remains. Then you should return to the ledge at the top of the ladder and climb a second ladder. It leads to a higher area with more tables. There are no waiting enemies here, just a mist-shrouded doorway. Step through the doorway.
On the other side, you'll come almost immediately to a 'T' in the path. Start by heading left and up some stairs, but move cautiously. As you reach the top of the stairs, look toward the right. There you'll see an area to the left where an armored soldier is waiting. Throw a firebomb at him or hit him with magic to make any conflict easier, since he otherwise can be difficult to fight in the cramped quarters.
Then you're ready to continue down a short hallway to the side. Note that it will come to another 'T' in the path and you don't want to linger long because soldiers will rush you from both sides of that 'T.' If you like, you can attract the attention of the guy to the right and lure him toward you, then kill him before you worry about the guy who waits to the left. The guy on the left is a knight with a cape. He's a new enemy and he packs quite a punch. He also has good armor, the Balder Armor, which you can sometimes obtain for yourself if you defeat him. Your best shot in combat is to wait for him to overexpose himself, then hit him with a flurry of slashes and hope your stamina lasts. Blocking more than one of his blows isn't really an option because he drains your stamina too quickly.
When you defeat the knight, head in the direction where he was positioned and look for a passage leading left. Head along that passage and you'll descend some stairs. At the base of the stairs and to the left, you'll find a more open area. Proceed cautiously and don't head too far forward. Veer left down the steps to find the Basement Key on a corpse in front of a portcullis (you can use it later to unlock the door at the base of the high bridge where you battled the Taurus Demon). Then look to the right and you can activate a lever to raise the portcullis, which creates a shortcut to your location (it's the area that was guarded by the boar).
When you're ready, head up the steps and as you do you'll probably alert another of the caped knights. Defeat him, then check the corpse to the left for the Halberd weapon.
Now it's time to ponder an approach on the area the knight was guarding. While you can head through the opening and almost immediately attract the attention of another caped knight, it's not recommended. Instead, head along the alleyway to the right (which you used a moment ago) and then take a left and continue along it. You'll head along the side of the cathedral area and as you come around the far side, the path widens. There, you can take on a group of weaker soldiers. Hit them from a distance with firebombs or magic if you like, while watching for a distant archer. Melee attacks also work, naturally, but they require you to go up against multiple foes at once and that's always risky. When those enemies are tended to, you can approach the cathedral through an opening along its back side.
Before you do that, though, head to the right. There's a long trail that descends in that direction. You can follow it to finally find a new bonfire in a room. Use it if you like, then note that you can proceed along a path to a locked door or you can descend. If you descend, you'll find a blacksmith who can repair or upgrade your gear. He'll also sell you reasonably-priced items that allow you to do some of your own smithing at bonfires and a less reasonably priced Crest of Artorias that can be used to break a seal in Dark Root Garden (it costs 20,000 souls). The variety of options makes this a wonderful area to farm souls (you can do so by battling the enemies above, just outside and possibly inside the cathedral) so that you can invest in upgrades. As for the path leading out of the room with the bonfire, it only takes you to a portcullis that presently bars entry to Sen's Fortress. Keep the location in mind for later, after you clear Quelaag's Domain, and return to the back entrance to the cathedral. Enter it.
Here, you will find a heavily-armored foe waiting just to the right. Approach so that he notices you, then quickly retreat to the area where you just took out the armored soldiers and archer. The knight should follow you and that's good because you need a lot of space in which to fight him and you don't want your battle to alert other enemies to your presence.
To battle this armored guy, draw out his attacks by moving around in front of him until you see him readying himself to swing. He has long range, so quickly roll back and out of the way. He may use a vertical strike that hits directly ahead of him, or he may swing wide. You can block both attacks with your shield, but doing so depletes your stamina and you'll need to quickly back away before any follow-up blows can crush through your defenses. Assuming that you dodge and the knight misses, you can roll forward and hit him a time or two before you retreat and repeat the process. When you have his health low, your foe will try to drink a potion to restore himself. Quickly move in to use another strike or two and finish the job. He'll drop a Titanite Shard.
Now you can enter the cathedral properly, though you should still move cautiously. There are some enemies to worry about and you still don't want to draw them to you until you're ready. Proceed into the building past where the heavily-armored knight was stationed to the right. You'll find a waiting altar with a corpse. Investigate the corpse to obtain the Fire Keeper Soul, an item that will improve the ability of your estus flask. On the wall nearby, you'll also notice a couple of elevators. One is open, so step into it and you can ride it down to Firelink Shrine for a brief but important detour.
Back in Firelink Shrine, head along the route that leads to the lower left. Make sure to use the bonfire so that your recent route doesn't need to be retraced if you fall in battle. You can also visit the man who is waiting in the side corridor. Talk to him and if he hasn't already, he'll give you a Copper Coin (talk to him two or three times if necessary). You can talk to him again after that and he'll offer to enter a Covenant with you, if your Faith is sufficient. This allows you to learn miracles. The man only has a few available, but they're darned useful. There's the Heal ability available for 4000 souls, for instance (it does what you'd expect and has five uses) and the Homeward ability for 8000 souls. The latter will return you to the last bonfire that you used.
Once you have invested in any miracles that interest you, should head down the steps that descend along the side of the area (just to the left of the place where the man is sitting on the short wall, not far from the bonfire). Down those steps, you'll find a lower area where there is a woman resting behind some iron bars in a cave. She's mute, but if you interact with her you'll be provided the opportunity to upgrade your estus flask. To do so, you'll have to use the Fire Keeper Soul that you obtained on the altar in the Undead Parish area, a short time ago. Updating your flask was your primary reason for descending to Firelink Shrine (besides the bonfire), so now you're ready to ride the lift back up to Undead Parish.
As you arrive back in the Undead Parish area, you're now ready to resume your exploration. Step off the elevator and head up the nearby stairs. Deal with the caped knight in the usual manner, then start into the room that he was guarding. Head forward and toward the right, where you'll note that the hallway enters a larger room. You should see windows and there are enemies moving in them.
You definitely don't want to fight all of those enemies at once. There is a whole mob of them. You can lure the first few toward you and fight them more easily, then let more keep coming and take care of them a few at a time. One of the enemies is a tall, cloaked magic user. He'll go down quickly once you're able to get to him and hit him with a few attacks, but rushing into the room doesn't make sense if you can help it. Tend to the enemies carefully and you'll have a better day. Note that fire attacks work well against them if they're stuck in the cramped hallway.
Once the room is cleared, you can explore again. To the left, there's a hallway that leads back in the general direction from which you just came. As you start down it, another caped knight will rush you. You can back out into the large room and deal with him where you have more space. Then head back down the hallway and climb some stairs. There, you'll find an area with a boarded over door to your right. Head beyond it for a moment and break the barrels to find a Humanity item. Then return to the boarded over doorway and break it down with your weapon.
In the hidden chamber, you'll find some stairs. Climb the stairs and at the top, you'll find a cell. There's a prisoner waiting inside. If you have the Mystery Key on you, it's possible to open his cell. Before you do that, though, make sure that you've cleared the area (including the lower cathedral and possibly even the outside areas) of any enemies. Then open the door and attack the prisoner. He'll get up and slowly start toward you, but soon he'll move more quickly You should retreat to an area where you have a lot of room to battle, such as the lowest floor of the cathedral or the area just outside of it.
The prisoner has a few attacks. If you try to attack him when he is guarding, he'll thrust aside your attack and skewer you with a sickle move that does a lot of damage. The best thing you can do is let him swing twice at you and miss. Then you can retaliate with a quick stroke or two of your own before backing swiftly away and repeating the process. Your foe is able to roll around and move quickly to strike you, which is why it's nice to have a lot of room in which to battle him. If you manage to win, you'll be able to pick up some Humanity and also the Ring of Favor and Protection, a powerful item that boosts your stamina and HP while allowing you to carry more items. If you decide to equip it, be sure that you won't want to remove it later; the ring breaks if removed.
Also, the time is right for a word of caution: if you free the prisoner but kill him, you'll miss out on the fantastic item that he carries. He'll eventually extinguish the bonfire in Firelink Shrine, but it is worth getting the reward later in the game. It's best to free him and let him live. If you leave him rotting in his locked cell he'll escape on his own.
When you're ready to proceed after the optional battle with the prisoner (you may wish to visit the local bonfire first), return to the room where you battled the magic user and his minions. Head across that chamber. There are two adjoining rooms on the back side. The one on the left contains a corpse that you can investigate to obtain a Large Soul of a Nameless Soldier. The area on the right side is more interesting. A red ladder ascends to another small chamber with a second red ladder. Climb that ladder to reach a mist-shrouded door. When you pass through the door, you'll be in the boss area.
When you first enter the battle with the gargoyles, there will be only one of them. You want to take advantage of that fact. Quickly get close to the gargoyle's body, near the base of its tail (not the dangerous tip) and start hacking away at the body or at the tail. You should be able to quickly inflict enough damage that the gargoyle collapses, and you shouldn't have to put yourself at terrible risk to do so.
When you kill the first gargoyle, or before you finish doing so if you're not quick about it, a second gargoyle will arrive on the scene. It approaches from the far side of the roof and it will breathe fire in your direction. Face it and use your shield to block it if you must, or roll out of the way as an alternative method if that's doable. Then you can move in close to attack before the gargoyle can try and toast you again.
Note that if you break the second gargoyle's face (risky, since it breathes fire), you can obtain the Gargoyle Helm. Another thing to note is that while both gargoyles are capable of inflicting heavy damage, neither of them have especially good defenses (this despite being made out of stone). If you have a good weapon equipped and your Strength rating is at a decent level, this should prove a decidedly easy battle.
Once the gargoyles are toast, head forward to the bell tower. There, you can climb a high ladder and work your way up the building to a lever at the top, just beneath the huge bell. Pull the lever to ring the bell, then carefully descend the way you came (you don't want to get in a hurry and accidentally jump to your death, after all). At the base of the bell tower, you'll meet with a character who identifies himself as Oswald of Carim. From him, you can purchase an item known as the Purging Stone. It costs 3000 souls and will cure you of the effects of Curse, a debilitating attack that enemies employ later in the game. If you have the funds to spare, you may wish to purchase several of the item and you may want to earn more souls and come back and purchase still more of the stones. You can also pick up Velka's Talisman for 5000 souls, if you're in need of a talisman.
With that done, you have a choice about how to proceed on your quest. You can head down to Darkroot Garden (an excellent option because it includes a sealed area where you easily can grind levels to your heart's content) or you can head back to Undead Burg. There, you'll find that you now have a key to unlock the door at the base of the stairs you descended after battling the Taurus Demon (at the end of the long stone bridge opposite where the red dragon is perched). If you use the Basement Key on the locked door to view a new portion of the burg.