Most of the time you're in a raid (unless you frequent PuGs) you're going to have a raid leader guiding you through the instance. They're going to give directions on what groups to pull, where to pull them, and kill orders for those groups. In a lot of cases (like mine) the Raid Lead is also the main Tank, and so this happens automatically and without a lot of overt instruction.
Raid Leads will often do extensive research into the various mobs you'll encounter in that raid. They'll read up on abilities and come prepared with kill orders and other such things. Barring this, they're hopefully good enough at adapting so that when they see the Ymirjar Battle-Maidens causing the tanks to bleed out faster than a BP oil well, they can adjust the kill order on the fly and make sure focus fire is used to down them quickly.
Dungeons usually don't have this kind of leadership going for them, though. And that's where you're likely to be learning to tank.
It's tough right now, because people have been used to over-gearing heroics for a long time. They're rampant AoE-fests with a few tank-and-spank bosses along the way. A lot of players have forgotten (if they ever even knew) some of the important details of dungeons.
For instance, did you that there are 3 Anub'ar Skirmishers in Azhol-Nerub? One in the pull before the Krik'thir gauntlet and 2 more in the left and center mob packs during the gauntlet? A few seconds after being pulled, the Skirmisher will Fixate on a random party member. During this time they cannot be CC'd or Taunted. It's an Enrage effect, so a Hunter or Rogue can get rid of it...but it's best to just burn that mob down first and quickly.
A lot of people don't know this. On more than one occasion I've been accused of being a bad tank who is incapable of holding aggro after a party member has died to one of these.
Moving on to Ahn'Kahet, do you know about the Ahn'kahar Spell Flingers? There's one in pretty much every trash pack in the first part of the dungeon...right up until you go down the long staircase past Elder Nadox and out of the Nerubian area. They're casters, so they won't pull to a tank unless their silenced or LoS'd. They have a rather nasty ability called Shadow Blast. It's very capable of one-shotting anyone not at full health. And if a tank (especially one that's still gearing up) eats one of these while it's getting beat on by several other mobs, a healer is going to be very hard-pressed to keep them up, if it's possible at all. They should be killed first in every pack, and kept interrupted until they're dead.
A lot of people don't know this. On more than one occasion I've been accused of being a bad tank after one of these has cause a wipe because I was obviously crit and I shouldn't be tanking heroics if I'm not defense capped. This statement, of course, has no less than 3 things wrong with it.
And while we're speaking of Ahn'Kahet, there's the the Twilight Worshippers down in the depths of the dungeon that cast Flamestrike. The damage on this spell is pretty lethal even to a well-geared group if people don't move out of it. And yet I've watched many tanks stand right in one with all the melee as the healer frantically tries (and fails) to keep them all alive. Or healers stand in it and not understand why they died when nothing was attacking them.
I could go on and on. List examples from every instance currently in the game. But you get the idea.
If you're a tank, especially a beginner tank, it's important that you know these things. Or that you find them out. For multiple reasons. Let's say people in your group aren't aware that the Skirmishers in AN need to be killed first. As a dungeon tank, a certain amount of de facto leadership falls on your shoulders. You can (often) help guide your DPS to do the things you want them to do. (More on this shortly.)
On the flip side, let's assume most of your group does know the Skirmishers have to die first and you don't. Your DPS is going to open on them while you get busy building main threat on one of the Watchers. Your DPS is going to pull aggro (pre-Fixate), and they're going to blame you when they die.
No pressure, right? Right.
So you need to do what you can to make sure everyone's on the same page. In the second scenario, this is easy. Educate yourself. Bam. Done.
(For what it's worth, I didn't research the NR dungeons before I started running them with my guild. We went in blind and figured things as we went. But it worked because it was early in the content cycle, everyone was learning, and we wanted to do it that way. It meant that we all had to pay attention to what was killing us. It was a lot more fun than looking things up on Wowhead ahead of time.)
The first scenario, though, is a little different. And this is one of those areas that can make good tanks stand out above the rest. A good tank well communicate with the rest of the group in several ways, essentially "directing" them into the actions he wants them to take.
The first way to do this is by attacking the targets you want your DPS to attack. Most DPS will attack the tank's target when they're not just AoEing everything. So if something in a group needs to die first, target it with your pull and spend at least the first 5 or so seconds of the fight attacking it. And if you don't get a good threat lead, stay on it to stay ahead. As soon as it dies, switch targets. Your DPS will likely be itching to pick up your next target so they can keep their numbers high on the meters.
The next is to use marks to denote high-priority targets. Most DPS will respect your marks and focus those targets down. There's several addons you can use for marking, as well as the default UI and keybinds that you can setup in your options. Don't go crazy and mark kill orders for every mob in every pack. It's unnecessary, especially this late in the content cycle. But if you know something has to die first, put a skull on it before you pull. If there's a second high-priority target, mark it with an X. More than that is overkill.
Third--and I know this one can be tough, especially in randoms--talk to your group. Actually use the chat window. Seriously.
You don't have to go into in depth conversations. You're not debating Chaucer or Ardol. Quick things like, "Focus Skull," or "Keep the Spell Flinger interrupted," are sufficient. Before you engage a boss say, "Pulling," and then wait two seconds to make sure your healer doesn't ask you to wait.
If you're still trying to get your feet wet as a tank, say so at the beginning of the instance. There's a right way to do this, by the way. Don't be apologetic, but don't over assert yourself, either. You want to communicate that your group shouldn't be expecting the same thing out of you as an experienced, ICC geared tank, but you don't want to come of as timid or uncertain.
- "I'm new to tanking and my gear is bad, so please don't get mad if I suck, okay?" = Too timid. You're begging any jerks in the group to jump on you and instantly blame you for anything that goes wrong.
- "DPS don't AoE. Attack my target only. And ffs wait 5 seconds for me to get good threat before opening up." = Too commanding. You're coming off as a jerk here and no one is going to respect your orders this way.
- "Just a head's up, I'm still gearing this toon and learning to tank. I'll do my best, but try to watch your threat." = Good. Most anyone that's not a total asshat will respect this.