Single target pulls: These are the easiest, usually. All tanks will, at some point, be given a single target taunt ability, and if that isn't available then there will be a big hit that "generates threat". Threat is what you want to generate! So, you have a couple of options open to you. Option one is to walk, charge, or saunter up to the target and whack them on the head, hopefully gaining their attention, and save your taunt in case one of your DPS is over-eager and grabs the target off you. Option two is to use your taunt to pull them. Taunting will cause the target to fixate on you for a short time, but only a short time, so don't taunt and think you're all sorted and can get a cup of tea. Taunt, then build "aggro" or "threat". How do you do that? Hitting them!
Multiple target pulls: These are a little trickier, simply because you have more targets to consider. They're often trash, so the first step is to figure out if any of them are casters.
Casters are often pretty obvious, thanks to their names. If one member of a trash pack is called healer, or priest, or anything that sounds magical, they're probably the caster. Why do you need to know which are the casters? Because, when you do damage to a caster, they don't come running towards you to bash you on the head. Instead, they stand on their own, casting. The trouble with this is that if you've got all the other mobs in a neat group, and are AoEing as you should, the caster may well get bored of shooting you with fireballs, and shoot your healer instead.
What can be done about this? Option one is to move the caster to you, either by gripping them, if you're a DK, or by interrupting them. An interrupted or silenced caster will move towards whoever they're targeting. Ranged interrupts are perfect for this. Your other option is to gain control of the melee group, and then take them to the caster. Keeping any kind of ranged damage on the caster helps a lot, too.
Other than casters, you need to do something that makes all the mobs hit you. AoE spells, so any spells that hit several targets at once, will always be good, as will any damage over time effects, even better if they can be spread to multiple targets. Try saving your taunt for any that escape, as taunting a single target in a large group will make them all run towards you, but you won't have much aggro.
Line of Sight Pulls: These come under the advanced pulls section, but can be a great way of stacking up several mobs in an easy-to-manage pile. You need two things: a wall or other obstacle that breaks line of sight, i.e. that you can't cast through, and a cooperative group. Leave your group the mob-free side of the obstacle, you go in and taunt or hit one mob, then run back around. Once you see them coming and they get in range, AoE like mad. If you have a ground-based AoE, drop it so they'll run into it. They're all stacked up for you! You definitely need a cooperative group, because if your DPS start hitting them before you are done getting aggro, this pull won't work.
Watch your group
Particularly when traveling through gates or doors, around corners or down steps, try to make sure you've got your group, particularly your healer, in range. If you leave a DPS behind, particularly in lower level dungeons, you'll be absolutely fine, but you won't survive for that long without a healer. Sure, you'll likely make it through one or maybe two pulls, unless you're a warrior, but not much more.
Also, keep an eye on your DPS and your healer, where they're standing, what they're up to. If you're getting more trash packs or adds than you can handle, it could well be that there's a "helpful" DPS or healer bringing them to you. This may be a help, but it may also be a hindrance. If it's a hindrance, say so, don't remain silent.
Watch your position
By this, I mean both your position in the dungeon or raid, and your position relative to the things hitting you. Think of your group as a ship that you're steering. Yes, other players have their own minds, but if you park a trash fight in the middle of a patrol's path, that's not ideal. Also, when positioning trash outside bad things on the floor, remember that melee DPS need to be able to get behind them. Move yourself out, and the trash out, and then keep going a bit further.
What's more, the reason melee DPS want to be behind their targets is because their attacks can't be mitigated nearly as much from behind by dodges, parries, blocks and the like. Guess what, the same is true for you! If you've got mobs stabbing you in the butt, you won't be able to mitigate that damage. Try to keep mobs in front of you.
My first max-level character was a paladin tank, and I wish someone had told me when I was new to tanking that it's OK to be new. It doesn't matter, we were all new once. If you've just landed in an instance that you've never tanked before, or don't remember, tell your group if you're not sure where to go. Chances are one of them will know. Do use the maps, too!
And the same applies if you're not going that fast because you're waiting for a patrol, or trying to figure out which one is the healer or spell-caster in the group, or trying to work out where you're headed next. If a DPS rushes off and pulls something, ask them not to, or simply explain that you're waiting for something to happen. Yes, you may get some muppet telling you to "go", but try not to get stressed about it. They're the muppet.
Lastly, if you keep losing aggro to a pet, be it a warlock pet or a hunter pet, ask them to turn off their pet taunt. They shouldn't mind!
And, if you're looking for more class-specific help, check out WoW Insider's tanking articles!
Visit the WoW Rookie Guide for links to everything you need to get started as a new player, from how to control your character and camera angles when you're just starting out, to learning how to tank, getting up to speed for heroics and even how to win Tol Barad.