There are 18 Bronze Trophies, 6 Silver Trophies, 6 Gold Trophies, and 1 Platinum Trophy.

Get all items in Normal or Hard Time Attack mode.
Hold down Square button at title screen

After beating the main game, watch all the credits all the way through. After they are finished you have the option to save your game. Save it and you will unlock both Hard and Time Attack Mode.

You must first complete the game once to unlock Time Attack mode.
To begin a challenge, walk up to a statue in the temple and pray (circle).

These items can be acquired through the Hard Time Attack Mode. You receive one item for every two Colossi you defeat in Hard Time Attack Mode.

Complete both Normal Time Attack and Hard Time Attack to receive all 16 items. Then, at the main menu, hold the "circle" button when selecting New Game, New Game (Hard Mode) or Load Game. Once the game starts, Agro will now be white.

If you use a memory card with an Ico save-game on it, your horse Aggro will have the Ico "I" logo on its forehead as opposed to the usual mark.

The white lizards crawling around in many areas of the game can be eaten to increase Stamina. Equip the bow and shoot the lizard, then press the Circle button to pick up and eat it.

Many of the trees within the game carry Fruit that can be used to restore and increase your Maximum Health slightly.
Equip the bow and shoot at the Fruit hanging in the tree, once it drops go over and press the Circle button to eat it.

You do not need to beat the game 4 times to reach the secret garden. All you have to do is familiarize yourself with the diagonal jump. When you grab a climbable wall (usually indicated by the moss growing on it), you have to hold jump and the left analog stick diagonally up until your character swings his arm, depending on what direction you tilted the analog stick. Once he does this, let go of jump and you will scale the wall diagonally without using a large amount of stamina.

Before you decide to reach the secret garden, it is advisable to complete about 7 colossi for the sole reason of acquiring enough stamina to take on this task. You could also search for white-tailed lizards as an alternative to gain stamina.

To reach the secret garden, you have to locate the wall of ivy on the northeast side of the temple shrine. If you use your horse and jump off of him towards the wall of ivy, you will save even more stamina. Now, follow it all the way up until you reach a ledge on the right. Go around it while still using the diagonal jump to save stamina. On the opposite side, leap across to the visible moss and keep going around the second ledge above. Once you get to other side, you have to go around yet another ledge, which is a little bit below your position. You will reach a circular pillar that you have to jump onto. This is easily done by situating yourself directly opposite of it and then jumping across. Hold on and keep going up. The moss will lead you a little bit towards the left to a small walkway. Follow the stairs to the secret garden. Enjoy, but do not eat the fruit, as it will decrease your health permanently.

After killinga Colossus, you can go back to their petrified bodies and by pressing Circle, your character will kneel down and pray. This opens Reminiscence Mode, a mode where you can find the Colossus again as though it were a flashback.