It is my first time in vanilla but ill tell you what i did in TLK when i first hitted endgame.
Basically you enter instances as DPS and do them for 2 or 3 times as DPS. Then you can lead it as tank since you know where to go and how to avoid needless mobs, etc.
And for tanking you need to know the basic rules:
1 - Keep the mob you are tanking with his back to your party. im not 100% sure but i think its easier to do damage from the back of a monster. But the advantage you get is that you know the mob is atacking you and if you lose the aggro you will see it turning around.
2 - Manage the Aggro. This is the amount of threat you cause to a mob. The higher the damage more threat it delivers, but warrior have threat related skills (not sure in vanilla since im just starting and didnt looked at the skills yet) so manage the cooldowns and get talents that icrease your aggro (if have any).
3 - Dont let anyone die! if you lose any aggro and the party have no off tank(a second tanker that helps you holding the aggro of minor mobs while you hold the boss) you must run to get the attention of that mob back to you.
4 - Be friendly and guide your crew in any way they need.
5 - If the healer die or get distracted its game over! No potion can hold an endgame boss! So be sure your healer knows he is THE Healer. But always with kind and friendly chat. Its harder to make people play their roles if you shout at them.
