How do I tank as a paladin?

My first suggestion would be to try to level some as your protadin in prot spec. See how it feels WITHOUT the other players joining in, then run an instance you already know as tank. (And gather gear first; while you don't need Defense Rating in 4.0, you don't want to be a squishy tank.)

• Avengers Shield to pull (or exorcism and then it, if you so desire; I could never be bothered)
• If you have full Holy Power, do a shield of the righteous or the self-heal, depending on your needs.
• Hammer of the Righteous (for AOE) or Crusader's Strike (single target); you'll end up using this skill pretty much every other time. This also applies a debuff to your actual target, so try to tab-around with it to make sure more than one mob gets hit.
• Judgement on cooldown. It restores your mana and, if traited properly, gives you a chance for a guaranteed crit from Shield. This also does a debuff, so again, try to hit different targets with it when applicable.
• Holy Wrath; note that it stuns certain types of creatures so don't do it unless you have them in place.
• Consecration: I mostly use only in crazypulls or if everything else is on cooldown and I have full mana. It's just not great any more.

My opening AOE rotation tends to be Avenger's Shield -> Hammer -> Judgement -> Hammer -> Holy Wrath -> Hammer -> Shield; I'll thrown in another Avenger's Shield if it procs again.

Some more generic tanking things:
- Your first goal is to survive. Your second goal is to keep attention off the healer(s). Your third goal is to do a lot of threat.
- A DPS meter may help; if one of the DPS is doing three times the DPS as anyone else, then that guy will pull threat from you. If he does it on a trash pull, it's his own fault so let him have them. (Ironically, I find it's the guy doing the LEAST DPS who is the most likely to pull threat).
- Keep moving, as much as possible. You don't have to worry about Rage or Runic Power depleting, and you DO have to keep an eye on your healer's mana bar; but you also tend to get better DPS if they know you're not pausing randomly.
- For the first few instances, if you're PUGging, tell the PUG that this is your first time tanking the instance. "Oh, he's new" is a much better attitude than "what a newb", and it lets the healer know how much attention he has to give.
- Use your tanking cooldowns and interrupts as necessary. DBM will help with that. - Learn who to prioritize and call targets when necessary. I have skull bound to X, so I can easily tell the group who to focus on when necessary. They generally don't listen, but that's not my fault, at least.
- It's better to hit a wandering mob with your Avenger's Shield as they get too close than to let the healer get their attention first. Being aware of everything going on around you is helpful. As healer or tank, I like to arrange it so that the tank is between potential adds and the healer, so if heals DOES get aggro, they run right through the tanks AOEs on their way.
- I mentioned it before, but keep an eye on your healer. If he's out of mana or drinking or otherwise not there, then don't pull.
- You can cleanse yourself to remove debuffs. Do so when applicable; your first goal is still to survive.

I never raided with my pally tank, but I did tank the heroics hundreds of times in WoTLK. I tanked enough that I was able to get the red protodrake from the achievement (can't remember the name of it off the top of my head), so I know more than the average person on tanking, but I wouldn't go so far as to claim that I know as much as any hardcore raider who tanks for a living.

I started in the same manner that you did: I rolled a newb pally and didn't start tanking until I hit 70. At that point, I actually returned to some of the dungeons in TBC and solo'd them so that I could get familiar with tanking. After doing a few normal runs I then switched over to some of the easy heroics and gave them a try. I then moved on to tanking normal instances in WoTLK, and from there to heroics.

Don't stress over having waited until 78 to begin tanking. I've known tanks that were horrible at it from day one, and I've known tanks who picked it up late in the game and were excellent at it! The same goes for any class. It all comes down to your aptitude as a player.

Lastly, I'm not 100% sure what the rotation is right now, because I've been too busy playing my shaman and mage in Cataclysm, but I know for certain that the rotation had changed significantly towards the end of WoTLK. For instance, consecration was the last in the rotation because A) it had been nerfed to the point that it generated the least amount of thread per second, and B) it was overly mana intensive, and C) the CD was lengthened significantly (30 seconds?). Your primary threat generator should be hammer of the righteous and you should be hitting it every time that it is off CD. However, take note that HotR is best for when you're tanking multiple mobs. If you are single target tanking then you should be using crusader strike. Avenger's shield isn't as high in the rotation anymore, but it should still be used when everything else is on CD. Oh, and don't forget about holy wrath.

Really though, google "pally tank rotation 4.0.1" and you'll get some good hits. Tankspot has some good rotations on it too. GL :)

Pally tanking isn't too bad, but it is different from every other tank class: Pallies are the only class where your threat generating capability drops as the fight progresses. Warriors and Bears both deal in Rage, rage generation is only a problem early in the fight. Likewise DK's with Rune Power. But Pallies generate threat via mana, and mana generation can definitely slow you down, especially if you're doing consecrate to generate AoE threat.

The best nuts and bolts numbers are at Elitist Jerks. Their protadin thread is so complete I'm not going to try and complete.

You have two taunts: hand of reckoning, which is a traditional single target taunt, and righteous defense, which is different in that you cast it on a FRIENDLY that has drawn aggro, to pull off the mobs. This will take some practice because most group taunts are AoE.

Watch out for mobs who have heavy magic resist spell damage reduction, or mobs that throw mana burn around. Since your aggro depends on mana, you can find yourself unable to generate threat.

Damage mitigation is where pallies shine. They are extremely hard to kill under normal circumstances. You don't really need to worry much here. Remember, if you bubble, you lose aggro in the worst way.

This brings us to the last point: DO NOT LOSE AGGRO. Warriors and Bears can pop an AoE taunt and be fine. They can also charge, to re-engage mobs that run off. Pallies weird targeted taunt is not nearly as cool, and when you lose aggro, and mobs start ping-ponging, you can very easily target the wrong friendly, and waste your only multi-mob taunt. The benefit of mana-based threat is that you can go nuts early, and hopefully build a good lead on your DPS happy partymates.

Avengers Shield and Exorcism are your long-range threat builders. Avengers shield is a pretty good tank tool: long range capt america style shield toss that hits 1-3 mobs. Good for a 3 pull, terrible if you only want two out of five or so. Exorcism you should know well from Retadin.