You can tank in any of three talent trees currently, there are changes planned for Cataclysm, which you can see here should you choose to. Please come back though because I'd hate for you to miss a gem of wisdom I might impart.
Here's a quick look at the differences in the trees;
Frost - early Wrath favourite and still a steady choice and my tanking tree of choice
Blood - better AoE tanking and high physical damage
Unholy - flashy, unnerving for some people to watch, but just as respectable (and fun :P) as the other two specs.
To have a look at specs, head over to www.deathknightbuilds.com and you can see them all laid out for you like a talent build buffet with glyphs on the side.
First things first.
My best advice, before you get into that dungeon or raid is to SWITCH TO FROST PRESENCE!! As a DK tanking noob, I frequently forget to do this simple and frankly, mandatory task. I've now got a mantra before raiding, Flasks, Fish, Frost presence.
If you're a new level 80 DK and thinking about tanking for the first time, my advice is to use those Emblems of Triumph to buy up T9 gear. At least a two piece kit will see you in good stead. You can use emblems to grab some i245 pieces too. Of course, in the interim, anything that drops from an Northrend HC and has tanking stats will be a good addition.
Unless you want to wait to grab an ICC 5 man weapon (or even Edge of Ruin* for ToC 5), I'd recommend getting a Titansteel Destroyer crafted. It's only i200 true, but it's a solid mace and is perfectly acceptable for running heroics and older Wrath raids, plus as it's crafted you don't have to grind for it, any BS should be able to whip one up for you. And the cost will be a lot cheaper than you might think. One note here. DKs are better equipped with a DPS weapon, don't ask me why. I've told you before I don't do the maths. It's what I've read across a few sites so that's what I've gone with.
Hmm let's think ... specs, gear ... what's next ... stats!
Def, def, def. Defense is your best friend. People talk about hitting a "def rating" and unlike things like hit rating, there is no real cap on your defense rating, but you do need a minimum level. 535 def rating will cover you for heroics, probably barring the ICC Heroics, 540 is the minimum for tanking in Naxxramus so is probably going to be your target for ICC Heroics. You can never have enough defense. As a DK, you can stick a the rune enchant of Stoneskin Gargoyle on your weapon which helps with defense, but you shouldn't use this rune to reach the def minimum. If you are struggling to hit the minimum def rating, pick up some Thick King's Amber to up it.
Following def rating, you're probably going to want to look at giving your green bar some oompf. Gems, specifically the meaty +30 stamina you get from Solid Majectic Zircon are now your best friend.
After defense and stamina, it's time to look at dodge and parry. For some reason I 've got 18% dodge and 22& parry as the figures to aim for stuck in my head but I can't find anywhere to back it up. I blame the hang over.
You'll not want to forget entirely about hit. Hit generates threat. That's why those pesky DPS can pull threat every now and then. So in between all the aggro your chucking at the mobs/boss, and all the crotch watching you're doing, between all the ducking and diving from the swinging axe, you've still got to land a punch every now and then. A little bit of haste and agility won't hurt so don't worry too much if you pick a smattering of those up. But please. For the love of the pixel gods, DO NOT EVER pick up anything that says "intellect" "spell power" "mana per second". You are not a healer. You are not a caster. You are a bad-ass-plate-wearing-hard-hitting-crotch-watching MoFo. In the words of Achmed, I KILL YOU.
So you want to prioritise
Def rating > Stamina > Dodge > Parry
Ok so I guess you're gonna want to know some kind of enchants to get now too aren't you? Not that you're greedy or anything. I've tried to include a bit of a selection where possible so you can tweak things as needed.
Head Slot -
Arcanum of the Stalwart Protector – Revered with Argent Crusade – 37 Stam and 20 defense rating
Greater Inscription of the Pinnacle – Exalted with Sons of Hodir – 20 dodge and 15 defense
if you're an inscriptor - Master’s Inscription of the Pinnacle – 52 dodge and 15 defense
If you are still lacking on def rating grab – Greater Defense – 22 defense rating
otherwise it's the old faithful – Powerful Stats
Frosthide Leg Armor – 55 Stam & 22 Agi
Jormungar Leg Armor – 45 Stam & 15 Agi
Tuskarr’s Vitality – 15 Stam & Minor Run Speed improvement
Greater Fortitude – 22 Stam
Expertise – 15 expertise rating
Major Stamina – +40 Stamina
Fur Lining – 102 Stamina - Leatherworkers only
Armsman – 2% threat and 10 parry rating
Expertise – 15 expertise ratingPrecision – 20 hit rating
Crusher – 44 attack power
Titanweave – 16 defense rating
Major Agility – 22 Agility
Superior Agility – 16 Agility
So I guess that's about it. If you think I've missed something please let me know and I'll stick another post together! Until then brothers and sisters, suffer well!
*Edge of Ruin is also known as that bastard axe. Because for months it didn't drop for me. Then I gets me grubby little mitts on it and two weeks later ICC arrives and this ugly bugger drops for me. Suckering suckertash.