This is the origin of the oft-heard phrase 'wait for sunders', three words so ubiquitous in the game that there are even guilds named after them. When you're tanking, if your group won't let you establish solid aggro on a mob, you simply won't be able to do your job. No warrior can tank without the support of his group or raid, because ultimately the entire time you are struggling to hold aggro on mobs, your party is generating aggro through damage and by healing, and while their aggro expands with the damage or healing they do, yours is static and based primarily on your tanking abilities, your damage being secondary. In essence, while tanking you are actually fighting with your own party for the monster's attention, and if they play poorly they can easily outstrip the threat you can generate to keep the monsters focused on you.
It almost... almost... goes without saying that unless you grossly out-level and out-gear an instance, you should be tanking in Defensive Stance and using a main or one-hand weapon and a shield. But while we've all done that instance run where the warrior didn't have a shield or hadn't trained D-Stance yet, I'll mention it anyway just to be safe. While even our humble correspondent has, at times, found himself tanking in an unorthodox manner, that doesn't mean he recommends doing so.
The primary abilities a tank should rely on are Sunder Armor, Shield Block, and Revenge (which will usually be used together), and Heroic Strike. Taunt should be held in reserve for when you lose aggro on a mob, as using it when you already have aggro on a mob is a complete waste of the ability. Taunting when you have aggro has no effect at all, save to put Taunt itself on cooldown, so only use it when you lose aggro or see a mob running towards the healers. All of the abilities listed above will be available to you by level 20 if not before, meaning that you can begin to practice your tanking by the time you're starting Ragefire Chasm or the Deadmines. A brief rundown of each ability I mentioned and how they work follows. Please note that I am using the base levels of the ability, and not the improved ones available through protection spec because this is intended to be a general guide for new warriors.
Sunder Armor lowers the armor of a mob you apply it to and also generates a high amount of threat. It's a good idea to get five sunders stacked on a mob as soon as you possibly can, and then refresh the stack once every 20 to 30 seconds to make sure the armor debuff stays that high. This will increase the damage your melee can cause while also helping to keep your threat high. Note that if you are in a group with a rogue, you will not be able to sunder the mob if they use their Expose Armor ability.
Shield Block increases your block rate by 75% for 5 seconds or until you block an attack, whichever comes first. As a result, it will not only greatly increase your own damage mitigation, but by practically guaranteeing that you will block, it allows you to use Revenge, a counterattack move which you can only use when you have successfully dodged, blocked or parried an incoming attack. While Revenge is not based on weapon damage and is never a particularly damaging attack (it was enhanced in 2.1.1) Revenge is one of the highest threat moves at a warrior's disposal and, barring instant attacks that require speccing into protection, is the best instant threat generator most warriors have. It's common for warriors to work to get five sunders placed on their target before switching to spamming Shield Block to force a block, and then Revenge for its high threat, refreshing sunders in-between this process.
Heroic Strike, which costs 15 rage and replaces your next attack, is useful for when you've got enough rage to spare and don't have Revenge or Sunder available for some reason. It doesn't rely on the global cooldown and does generate significant threat, although inferior to the other abilities already mentioned. If you're swimming in rage, it's not bad to throw a few HS into the fray. Some warriors swear by a very fast tanking weapon because it allows them to use HS more effectively, since HS is not an instant but rather replaces your next attack.
Okay, we've covered some basics. There's a lot more to warrior tanking than just these, however. Next time we'll talk about how the other classes can help you with abilities like Misdirection and Sap and how to itemize your fledgling warrior for better tanking as you level.