Beginner's Guide to Tanking
Role of a Tank
A tank's role is mainly to hold agro. In addition to this, quite often a tank will be selecting kill order, marking, pulling and making sure to pull agro off the healer when needed. A decent tank keeps a group alive. A crappy tank kills the group. An awesome tank is someone everybody loves.
Before pulling, always make sure your group is ready for a pull by asking "Rdy?" If a healer it low on mana, wait for them to regen. If someone has gone AFK make sure they are back before pulling.
When you are marking always be sure to specify kill order. Don't assume that everyone uses the same set of marks. While there is a generally accepted order to marking, just be on the safe side and specify. That way everyone in your group is on the same page.
Be sure you know your class's abilities for generating threat. Threat allows you hold agro. Different classes use different abilities for this. Quite often abilities that cause a lot of damage will help your threat.
Be sure to keep the mobs in front of you. You can not parry, dodge or block an attack that comes from behind. If you are concentrating on one mob in front of you while three mobs beat on your back you're going to make it tough for the healer. Back up towards your group so that all the mobs are in front of you and you can see if one runs past you to attack a group member.
Pay attention to the mobs around you. Don't pull group of mobs in the middle of a patrol's pathing area without killing the patrol first. Don't stand and beat on a meelee mob while a caster is standing back and taking out your healer. Also make sure to mention to group mates that if they have agro, run to you, not away. This will keep you form losing threat by trying to chase down mobs.
Before you say you can tank have the right gear and the right spec. If you're a warrior or a paladin, you'll need a sword and shield, not a two hander. It also helps if you are protection specialized in your talents. Don't think that if you wear plate and have a shield or do a lot of dps, it means you can tank.
Be sure to take time to tank in groups while you are leveling. Don't hit 80 and expect to tank if you've been solo dps for the last 30 levels. Offer to tank lower level dungeons so you can get in some good practice in an easier environment. It takes practice to be a good tank.
Stats and Spec
One of the main stats for a tank is defense. The more defense you have, the less damage you take. This is important since your goal is to take the most damage for your group. Make sure you have plenty of defense and stamina. You'll want a good number of hit points before trying to tank. Other skills are parry, block and dodge. To some extent the amount of those you want will depend on your class. If you're a feral druid, block won't do you much good for example.
Be sure to take a bit of a time to research your class. Know good talent specs, abilities and desired stats. Be sure you have the correct talent spec for tanking. Don't try to tank as a holy paladin unless you're level 50 and running the Deadmines. Each talent spec meant for tanking gives special skills that can make you live longer, generate more threat, increase your hit points and so on.
When you mark you are selecting a kill order. That means you kill first, second and so on. Be sure to specify kill order before pulling. It's much easier to maintain agro and make things easier on the healer if you set up a kill order, even if you are using AE to kill the mobs. It also keeps things organized and organized groups are more effective. Quite often there will be certain mobs you need to kill first such ones that heal or summon other mobs. Marking allows you to make sure they die first so they don't cause more trouble.
To mark, right click on a mob and use the raid icon menu. You must be the group leader in order to mark. Here is a list of marks from Beginner's Guide to Grouping.