Before we get started, we'll link back to our discussion of hit and expertise for tanks and DPS. Generally speaking, we can say that for level 80 non-heroic instances you want to be at least at 20k health unbuffed in your tanking set and as close to 535 defense (the amount needed to remove critical hits against a level 82 mob, which is the highest boss mob to be found in 5 man instances) as you can get. Other stats, while important for threat and survivability, can be worked on after hitting those targets. To be fair, you can most likely start tanking non-heroics at 530 defense and be fine. Some bosses like Volkhan (as an example) seen to drop to level 80 or 81 in non-heroic, meaning that the defense cap for a level 80 tank to be uncrittable against them drops as well.
Also, if you're at 21 to 22k health unbuffed but a little shy on defense, it's not a major issue. You can survive the occasional critical hit. As long as you're within three levels of the highest level mobs you're going to face (so if you're a new level 80, you're golden, no mob will be more than 2 levels above you) you won't be taking any crushing blows, so the spike damage from a critical hit every once in a while won't be as horrible to have to deal with.
One thing that should always be pointed out, especially to up and coming tanks, is the importance of gear selection for specific fights. We mentioned it back in BC, but it bears repeating now: you should have gear for threar, gear for avoidance, gear for mitigation, gear for stamina. Sometimes some pieces will be good for more than one (in fact, it's almost always the case that tanking gear will at least have stamina and defense on it - I say almost always because one of my current tanking rings is Gatekeeper) but the point is, don't be afraid or reluctant to switch gear between fights, or even between trash pulls if the demands of the pull are going to be different.
Don't be afraid to tank in non-heroics even if you're just starting out at the bare minimums. As long as your party is motivated, knows what they're doing and can be responsible with threat these instances are designed for a group of players to gear up in. You're expected to wipe a few times (in general, you'll need to have a good attitude about wiping as you start out at 80, be you tank or DPS) but as long as you compensate for your gear with reasonable use of abilities and understanding the fights (Don't pull General Bjarngrim when he and his two adds are big and shooting lightning everywhere, for instance) you can complete this instances in quest blues and crafted gear.
We've covered gearing up for Wrath before: here's our general round up of weapons, and here are the four posts we covered everything else in, parts one, two, three and four. For an up and coming tank, I would advise you that the factions to pay the quickest attention to are the Wyrmrest Accord (immediate tank cape at honored and a solid tank chest and boots at revered) the Argent Crusade (tank legs at honored, tanking head enchant at revered) and the Sons of Hodir (tanking shoulder enchants at honored and exalted). Blacksmiths will want Titanium Plating, which requires exalted with the Alliance Expedition or Horde Vanguard, but the rest of the gear is uninspiriing at best, and the Kalu'ak offer a decent chestplate at honored but in my experience just doing the starter quests in BT or Howling Fjord and Dragonblight will get you to honored, so there's no need to grind the rep.
As for heroics and raiding: aside from 535 defense for heroics and 540 for raids and 163 hit rating (5%), keep in mind the rule to accumulate more than one set. You'll basically want 22k health unbuffed (including no commanding shout) for heroics and 10 man raids, and about 24k for 25 man raids to start with. Expertise doesn't really ever get bad for a tank, but you also have so many other stats to consider (dodge, parry, block, pure defense) that you really don't want to stack expertise over all else.
We've covered raid gearing, so I won't go too much more into detail since this is intended more for the new 80. Also please keep in mind the fluidity and flexibility of all of these statements depending on the rest of your group. I've tanked heroics on my tauren in gear that had significantly less health - I tanked Heroic UP with 20k health unbuffed) because my healer and DPS were all 10 man raid geared to the teeth. Then I PuGged heroic AN and got splattered the same day. Some of that was the lack of anyone in the group who could competently cleanse poison, but if I had been better geared it would have been easier to keep me up.
In general, 5 mans and heroics are meant to be forgiving as long as you bring a reasonable level of ability to them, and even raiding right now is pretty forgiving of the new player as long as he or she is reasonably skilled, has certain bases covered (you can tank in 10 man Naxx at 28k health buffed as long as you're uncrittable and know what you're doing with your cooldowns and have a decent set of healers) and doesn't stand in anything.
Seriously, World of Warcraft will teach you to fear floors. Is that a new spot on the floor? Run from it as fast as you can it brings death the floor is the architect of pain and despair aieeeee. Not that I'm so sick of Void Zones at this point that I could vomit blood or anything.
The reason I've spent so much time talking about tanking targets and haven't really covered DPS is because of a degree issue. Tanking is much more drastic in its "You must be this thick to tank this boss" bar, where the DPS ride-marker is more forgiving in its way. If you are not geared well enough to tank, you'll die. If you're not geared well enough to DPS, you'll do crap DPS and depending on the rest of the group, you may die or you may kill everything and just be embarrased every time someone posts a DPS meter. This doesn't mean you shouldn't gear your DPS warrior to the best of your ability or do your best to do good DPS. And it is possible as a DPS warrior to do competitive DPS without the best in slot gear in every slot, depending on the gear of the other players around you. But in general, DPS gearing is much simpler.
We've covered hit and expertise but it's worth mentioning again that for DPS warriors, you only want enough expetrise to remove dodges. Any expertise above the amount needed to remove the boss' 6.5% chance to dodge is wasted for DPS (while tanks are trying to remove parries as well, meaning it's a much stronger stat for them) while the white hit cap for fury is ridiculously high (it would be something like 885 hit rating) so you're certainly not going to stack hit as a new 80 to that extreme. It's not possible with the blue gear available, and even trying for hit over everything else would lower your DPS too much. Arms needs roughly 263 hit rating to be capped (since they're using a single 2h weapon) and fury needs 164 hit rating with full Precision, which you can take points out of as you get more hit on gear. Expertise depends on the tree, as Arms has Strength of Arms and fury currently has Weapon Mastery, but once 3.1 comes out arms will have SoA and Weapon Mastery, meaning fury will need 26 expertise and arms will need 14 expertise.
Right now Strength/AP is probably the best pure DPS stat for warriors, stronger even than crit rating. As long as you have enough hit to ensure that your special attacks are landing, and your crit rating is reasonable, strength is a very, very strong DPS stat for warriors. You want more of both, of course, but my advice is to prioritize strength: on my human I've had to regem to gain more Strength and reduce my crit down to 37%, but this isn't the kind of problem you're going to have as a new 80. Still, it bears repeating: once you're in decent shape with precision and weapon mastery to hit the hit cap for special attacks and to push dodges off of the table, you should be looking for Strength, then Crit. You'll find as your gear improves that you'll be able to move talent points out of precision and weapon mastery, but for starting five mans you'll probably want to have both maxed to give you room on your gear choices. Please don't take this to indicate that crit isn't good, and some will tell you it's just as good as Strength, so please test it out and choose your own preference. Also, keep in mind that Strength on gear scales with buffs in a way that pure AP does not, so go for STR over pure AP unless the pure AP is too good to pass up.
I can't give you a hard and fast "you must have this much AP/Crit to start five mans" because, well, the whole point of running five mans is to get geared up. It would be nice if you have at least 2k AP before battle shout and crit rating in the high 20's - factions you can look at as a DPS warrior for gear include the Knights of the Ebon Blade (DPS head enchant and solid 2h sword plus a heck of a set of DPS boots at exalted), the Argent Crusade again (they have a decent DPS cloak and a 3.6 speed 2h mace) and the Sons of Hodir for enchants and a solid set of DPS shoulders. Wyrmrest offers these legs, which are reasonable, but to be honest I've always gotten better before I even got to revered with them.
Okay, this is a basic overview for now. I may sit down and cull my older gearing posts in the light of the Argent Tournament coming out in 3.1 and us knowing more about where some drops occur than we did back when I wrote them last year for next week.