Preparing to Tank
First, If you haven't read Allison's Open Letter to Death Knight Tanks, you should now. There are some very solid basic rules about pulling and keeping aggro that any serious tank should be familiar with. Once you're back from that, we'll continue.
The first thing to wrap your mind around is that every tree can tank, at least to the 5-man Heroic level. Blood, Unholy, and Frost are all viable choices to stand up against a big bad boss or a group of elite mooks. That said, you still need to specialize if you're serious about tanking. Essentially, each tree has some solid early tree tanking talents, such as Blade Barrier, Anticipation, and Toughness, which almost every build should probably pick up. Then, the tree you specialize in has late tree talents such as Vampiric Blood, Bone Shield, and Unbreakable Armor.
The final thing you need to consider is your gear. As awesome as it looks, the newbie Death Knight quest gear is not for tanking. You need stuff with more AC, more Stamina, and maybe a bit of defense before you can get seriously about tanking. Your healers will thank you, and your repair bill will thank you.
You can start gathering tanking greens as early as Hellfire Peninsula, with quest rewards such as Flintlocke's Piloting Pants and the Underworld Helm. Of course, by the time you're ready to tank Utgarde, you'll hopefully have found more solid gear. I myself saved a selection of "Of The Champion" greens from my pre-BC dailies, and there should be a few good solid tanking quest rewards from Howling Fjord or Borean Tundra you can grab, such as the Blacksoul Protector, Amberplate Leggings, the Rigid Tuskring, or the Cragthumper.
And yes, I'm still recommending that you grab a 2-handed weapon for tanking, no matter how tempting it might be to grab a Sword of Heartwrenching Slaughter and a Blade of the Empty Void. Any extra defense or parry skills you pick up will still be negated by the extra attacks the mobs get in when they parry you.
Admittedly, it's going to be hard to compete for a tank slot against old tank characters who have had time to build up a set of tier 6 tanking armor that breaks 20k health and already have enough defense to make them uncrittable. Still, you should be able to find enough odds and ends that you can pull through a dungeon and not be too much of a pain to heal.
So once you have a good set of high-stamina, high-AC tanking gear, and once you've learned how to pull, where do you go from there? Let's look at Death Knight tanking in action, then check out a few talent builds.
Using your Skills and Talents
So once you're in a dungeon, how do you handle tanking? First of all, make sure you're using Frost Presence. Not only is the extra armor helpful, but the threat boost is pretty much required to tank effectively. After all, your only specific "high threat" ability is Death and Decay, and your taunts have relatively restrictive cooldowns. You'll be putting out the damage to try to stay on top of things, and you'll need that extra threat.
So as you might expect, your basic damage rotation in tank mode is probably not going to be too different from DPS mode. In the most basic, you'll be gathering a bunch of trash mobs, throwing out Death and Decay, apply Frost Fever and Blood Plague, and use Pestilence and Blood Boil to spread the damage love around, adding in runic power dumps where needed and finishing things up with an Obliterate or its talented equivalent.
Of course, you'll want to keep on top of threat, and when a monster gets loose and goes for the healer, be able to make a quick judgment as whether to use Dark Command or Death Grip. While Death Grip is nice in that it will pull the mob to you, it also isn't a true taunt. Once those 3 seconds are up, the mob will go back to attacking whoever is on the top of its threat list. If that isn't you, you're back to square one, threatwise. Dark Command, on the other hand, will put you right back at the top of the mob's aggro list.
The other thing to manage as a Death Knight skill user is your triggered defensive skills. Every Death Knight will have Icebound Fortitude and Anti-Magic Shell. Frost Death Knights will have Lichborne and Unbreakable Armor, Unholy Death Knights will have Bone Shield, and Blood Death Knights will have Vampiric Blood.
Essentially, you probably aren't going to get away with spamming the defensive key. That means you have to get a feel for when you're going to see the damage spikes and learn to save your abilities for those. If you know your target enrages, perhaps save at least one of your triggered skills for an enrage. You should also make yourself familiar with Empower Rune Weapon. If your defensive abilities are on cooldown at a crucial moment, being able to use Empower Rune Weapon to refresh them could save your group a wipe. With Anti-Magic Shell, you'll need to watch the enemy's spellbar like a hawk, activating Anti-Magic Shell only just as the spell has finished casting to make sure it's up to absorb the damage.