Blood Deathknights should be spending their runes to use Heartstrike (which they get as their Blood Specialization) and Deathstrike (which is the focus of their Mastery specialization).
Talents
Here is an example talent spec: http://www.wowhead.com/talent#jcbG0srMusd
The core talents you want to be sure to pick up are Blade Barrier (reduces damage taken), Toughness (increases your armor), Bone Shield (tanking cooldown), Sanguine Fortitude (improves Icebound fortitude, a tanking cooldown), Rune Tap and Vampiric Blood (both tanking cooldowns) Improved Blood Presence (Crit immunity), and Dancing Rune Weapon (survivability AND threat cooldown).
After that, you have a choice between dipping into either Frost or Unholy. Unholy is almost always going to be strictly better, as you do not want to dual wield to tank (dual wielding has an inherently higher miss chance than 2-handed weapons, so your threat is going to be much worse) nor are you using obliterate (as you should be using death strike). Unholy gives you your choice of reduced taunt cooldown (Deathgrip), free +hit% for your spells (so deathcoil won't have a chance to miss) and morbidity (boosts the damage and thus, threat, of your runic power dump: deathcoil). Taking frost as far as Lichborne, however, means you can activate Lichborne, and then cast deathcoil on yourself to get a free heal if you're low -- a neat trick, and handy if you're low on health after a big boss burst.
Glyphs
Prime Glyphs:I would go with glyphs of Death Strike, Heart Strike, and Deathcoil for your prime glyphs, though depending on playstyle you may prefer to swap the latter for Runestrike (though I'll always take +damage over +crit chance, since the former is more consistent) or Death and Decay if you want additional AoE threat.
Major Glyphs: Dancing Rune Weapon is a no brainer, but beyond that your choices highly depend on your playstyle.
Minor Glyphs: I'd Glyph Blood Tap and Death Grip, but these are minor glyphs, and won't have much effect at all in regards to your tanking.
For further discussion, I'd point you toward the Elitist Jerks' Death Knight Forums, specifically the tanking thread, which can be found here.
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.
Best Answer
It's quite a short answer as you don't have many skills at level 15.
First off: There is no bonus threat anymore. It's now passive for all tank specializations. Monks used to have Dizzying Haze for that, but it was removed in Legion.
Your taunt is Provoke. Use that on bosses or other mobs that are out of melee range. Use Roll to get to the mobs faster when your taunt is on cooldown or you need it for another mob.
With higher level it will be easier to keep aggro. You'll get more skills as you level up.
Especially in low level dungeons the damage dealers are often reckless and pull too many mobs or just attack the boss without you being ready. If they die, it's their own fault (and as a tank I might just then take a 10 seconds coffee break when someone keeps pulling mobs without my permission).