As with any MMO, combat is going to be a central part of your characters life; this section is intended to give you some pointers on how to proceed!
Know Your Limitations
Every character is going to have different soloing abilities, depending on its class, equipment, and the skill of the player. If youre a total pro and have made multiple runs through the instanced dungeons appropriate for your level to outfit yourself with blue loot, then you can probably take on mobs a few levels above you; casual players will have to settle for a bit more parity in their targets. The reason for this is that monsters are drastically harder to hit with weapons and spells when they start exceeding you by two or three levels. If you have fantastic equipment and can deal more damage than most other characters of your class, then you may be able to overcome these drawbacks, but in general, youre going to want to stick to mobs that are no more than two levels above you, especially when youre just killing for experience.
If youre coming from other MMOs where you have to manually check a monsters difficulty with a key, then youll have to adjust to the fact that WoW gives you a straight numerical readout of a given monsters level; the comparison between your level and that of your target will give you a ballpark indication of the difficulty youll face if you try to take it down. If your level is higher than the monsters, then all of your attacks and spells should hit, save for when the game calculates an automatic miss or resist, which will occur on something like five percent of your attacks, or when youre using a weapon that you havent skilled up.
Almost any class should be able to solo an equal-level monster without a problem, and some will be able to take on two or three at a time without needing to flee. Most of your attacks and spells will connect, depending on any specific defenses of the target. (A frost-resistant monster will start to more readily resist your frost spells as its level approaches yours, for instance.) Of course, these enemies will deal more damage than lower-level monsters will, so youll have to keep an eye on your health meter while fighting, and proper combat techniques (rooting, kiting, using pets to hold aggro, etc.) will become critical.
Know Your Targets
Whether youre going after quest mobs or just grinding out the last few thousand experience needed for the next level, its important to know what youre looking for, and pick your targets accordingly.
So far as quests go, theres a lot of potential for confusion due to the way quests will specify a specific monster type; when this occurs, you need to make sure that youre going after the correct targets. For example, if a quest wants you to retrieve items that drop off of Snapjaw Crocolisks, then youll find those items only on monsters called "Snapjaw Crocolisk." If you go around killing Saltwater Crocolisks or Sawtooth Crocolisks, youre not going to find what you need. Even if you kill something called an Elder Snapjaw Crocolisk, youre still not going to get the items required for your quest. Thus, you need to be absolutely sure that youre killing the kinds of mobs required for your quest items to drop.
Generally speaking, quest items will drop on every third or fourth mob that you kill of the specific type, so if youve killed twenty mobs of the same name and havent gotten what you need, double-check your quest to make sure youre going up against the correct enemies. (The drop rates are often much lower for items that you only need one or two of, though.) If youre having trouble finding the mobs for a given quest, ask for help in the general channel and youll hopefully be pointed in the correct direction!
If youre just looking for experience, though, then youll obviously have more options available to you. What youre really looking when you solo for experience is efficiency, meaning that youre aiming to maximize the amount of experience that you gain within any given amount of time. Obviously, monsters a level or two above you will give you more experience than those a level or two below, but theyll take longer to kill and will require you to rest more often. You want to find a nice sweetspot here, so that you can consistently be killing foes without having to constantly eat food after each fight, but different classes will obviously have different groups of foes that they want to target, based on their own peculiar strengths and weaknesses. Mages, for instance, will probably want to go up against other ranged enemies, as melee mobs will cause problems when they close in and start attacking.
In addition to just finding mobs that are easy to kill, but which still give you experience, you also have to factor in the mob respawn rate and popularity of the area. Even if a group of mobs are giving you great experience without much effort expended, theyre still not going to be of much use to you if you have to run around for 30 seconds in between each mob. Whether this is because the mobs are on a long respawn timer or because there are a dozen other players camping the same mobs doesnt really matter; if either case is in effect, its going to hurt your experience-earning efficiency. For this reason, youll probably be forced to avoid camping mobs that are also needed for quests, especially on busy servers during the peak gameplay times (afternoons, evenings, and weekends). Blizzard has been pretty good about either increasing the respawn rate or increasing the number of mobs when theyve become overcamped, though, so perhaps this wont be as big of an issue in the shipping version of the game.
Lastly, youll want to maximize the side benefits that you receive from killing enemies. Earning experience is great, sure, but earning experience and gold at the same time is even better! If youre killing around, note the kind of drops that youre getting, and whether any mobs are dropping items worth vendoring. Humanoid enemies are particularly noteworthy, as they always drop some amount of gold, and will often drop cloth which can be used in First Aid or sold for more cash, or healing potions. Beast enemies will occasionally drop valuable items, usually in the form of some body part, which can often make them just as lucrative as humanoids, albeit with the added hassle of having to carry a bunch of bones and effluvia back to a vendor.