Whenever I ask my friends what sort of topics they'd like to see covered on a Death Knight blog, I almost always hear, "Tell them how to tank!" I came into DK-ing from playing a Prot Warrior in Burning Crusade, so I had already learned the basics of tanking back when "threat" was as much about yelling at your shaman friend who thought Chain Lightning was the best thing ever as it was holding the attention of the mobs. But for a lot of people, their Death Knight is their first tanking class. If that's the case for you, well...welcome to the wonderful, stressful, but ultimately satisfying world of tanking!
Let's start with the two most basic aspects of tanking: Threat and Mitigation.
Since patch 3.0, generating threat has been much easier than it used to be. Until your DPSing friends start pushing at least 4k DPS you'll likely hardly ever bother to glance at a threat meter. But it's still important to understand the concept, because to hold threat first you have to establish it.
Threat is, essentially, based on damage. So how do we generate more threat than the DPS if we do less damage than them? That's thanks to our good buddy Frost Presence. The most important thing you as a Death Knight can do to generate threat is to stay in Frost Presence! In addition to other benefits, Frost Presence makes the little bit of damage we do much more threatening than, say, your mage friend's damage. But you can't just turn on Frost Presence and stand there. Threat generation is why you will see tanking armor with DPS stats like Hit on it and why so many talents are shared between a good tanking spec and a good DPS spec in the same tree. Though I focused primarily on mitigation talents in my prior post regarding tanking talents, the need to generate threat also makes DPS talents like Bladed Armor excellent for tanking. This is why Threat of Thassarian, on the surface a DPS talent, made DW Frost tanking not only viable but the preferred tanking spec of the moment. All the mitigation in the world won't do any good if you can't hold the boss' attention well enough for the DPS to do their job.
When tanking, one should keep in mind the need for both single-target and AoE threat. Single-target threat is most important on bosses and on those rare occasions when DPS can be coaxed into focusing fire. However, whenever engaging more than one mob you always need to employ AoE threat tactics for one very important reason: so your healer doesn't get eaten. The most important tool Death Knights have available for AoE threat is Death and Decay, and I personally recommend it in any situation where you're pulling more than one mob. It's also a good idea to hit Pestilence as soon as your diseases are up on your main target to spread your DoTs around, and then of course Howling Blast if you're Frost. Death and Decay alone is enough to keep mobs off your healer, but a full disease and HB rotation will give you enough threat on the whole group to let the DPS say "lol kill order". As someone who started tanking as a warrior, nothing makes me happier than to see the DPS kill something I've held aggro on exclusively through AoEs.
And for any class, no matter your level or your gear, the single most important thing you need to do to establish and hold threat is pay attention! Death Grip or Strangulate that caster who didn't run into your Death and Decay with the rest of the pull. Taunt (in our case, that's Dark Command) that mob running off to smack the mage around. Situational awareness isn't what makes you a good tank, it's what makes you a tank, and you can never learn it too early or practice it too much!
The other basic aspect of tanking is mitigation, or your ability to take punishment and stay alive while not making your healer cry. This is a much more passive thing than threat. You will have abilities that you can use to decrease the amount of damage that you take based on your level and spec, but primarily your mitigation will be tied to your gear and your talents. I don't like bogging down my DK101 posts with too much math because I don't want them to be obsolete in a month's time - that's what the links to Elitist Jerks and Tankspot on the sidebar are for - but once you reach 80 there will be some vitally important numbers to remember.
While leveling, though, there aren't many benchmarks for mitigation, so the most important thing (apart from, again, Frost Presence) is to be as well-geared as you can manage. I can vouch for the fact that a DK in an Unholy DPS spec can get away with tanking Blood Furnace in starter gear and plate heirlooms because I did it once for a few pulls when the tank DCed, and my healer friend said it was no trouble keeping me up. But if you plan to tank as you level, put points into a proper spec for it and grab every quest reward with Defense you're offered. Greens with "...of the Champion" stats are also excellent. You don't need to have every slot filled with tanking gear like you do when you're 80, but the more you have, the better. Make sure all your armor is plate. You may have found some awesome leather gloves for DPS, but no one but a druid should be caught trying to tank in anything but plate. And that goes for any heirlooms you may have, too. Don't try to tank anything in cloth heirlooms! I saw a paladin try it once and it was not pretty.
Anything I have to say about level 80 mitigation will be completely obsolete once Cataclysm comes out, but for now, if you decide you want to start tanking at level 80, make sure you have at least 535 Defense Rating for Heroics and 540 for raids! That's Defense Rating, not raw Defense. This will keep you from taking massive critical hits. You also want items with Dodge and Parry for mitigation and Stamina for straight-up HP. Since Death Knights don't use shields, we don't benefit from Shield Block. In general, it's best to have as much of those primary mitigation stats as possible, especially as you're starting out and getting a feel for the role.
Balancing the Two
I often see newer tanks who are still getting a good grasp on these concepts asking, for instance, if they should use DPS weapons vs tanking weapons when dual wielding. This is an excellent example of deciding between threat (DPS weapons) vs mitigation (tanking weapons), and often these decisions are situational. If you're running Heroics in full T9 or T10 with equally geared DPS and heals, then you want to focus on threat so you can stay ahead of your friends. If you're just starting out in Heroics or with a group running earlier Wrath raids and your DPS friends aren't riding your tail on the threat meters yet, you might as well make life easier for the healers and focus on mitigation. Sometimes it's even good to put a nice slow hard-hitting DPS weapon in your main hand and keep a tanking weapon in your offhand for the mitigation stats.
With these basics in mind, I encourage anyone interested in tanking with their Death Knight to use the aforementioned resources to find the most up-to-date information on the specific numbers they want to be aiming for as of the most recent patch. Then check that Tank role, queue up, and good luck!