3)  How to tank

Basic idea - you want to be hit the hardest, the most frequently, and by the most mobs.  Moreso than anyone else.

How to do this:

#1 - be in Frost Presence.  Your threat generation is dramatically increased (threat being a combat characteristic that everyone generates - tanks make more than anyone else), your stamina is increased, the damage you take is decreased, and your armor is increased.  You won't generate enough threat if you try to tank in Blood or Unholy Presence - particularly since you'll be much easier to kill.

#2 - Is a mob/enemy attacking someone else?  Use Dark Command or Death Grip to taunt them off and stick them back on you.  Dark Command has a 7 second cooldown, Death Grip has a 35 second cooldown.  Both act as taunts.

#3 - stats.  First one you want to "cap" is defense.  You need a Defense skill of 540 to be "uncrittable" by NPC mobs that are "boss" level (lv. 83).  A Defense skill of 535 will make you uncrittable by lv. 82 NPC mobs - the highest you'll see in a 5-man heroic dungeon.

To reach 540 Defense skill, you need to acquire 115 Defense skill from gear (you'll get 400 from being lv. 80, and 25 from using the Rune of the Stoneskin Gargoyle - and its variants - on your weapon).  115 Defense skill translates to 566 Defense rating (4.92 Defense rating = 1 Defense skill).

After reaching the Defense skill cap, you'll want to think about keeping your Hit rating and Expertise rating up to par.  The hit rating cap is 263 rating (+8% chance to hit, you can't miss) and the expertise rating is 173 (with talents, translates to an expertise skill of 26 - means NPC's can't dodge your attacks).  If you reach both caps, you will only miss your target with melee attacks 6.5% of the time (due to parries).  It's advised to get close to both caps, however it's not 100% necessary and it's generally only reachable at endgame gear levels.

Beyond Defense, hit, and expertise - you need loads of stamina, dodge, and parry.  Generally speaking, your gear will have plenty of stamina so don't worry too much about that.  However, if given a choice between dodge and parry, always go for dodge - Death Knights swim in parry due to our Strength/Parry conversion, and parry reaches its effective skill cap very quickly for us.

#4 - Gems/enchants

Here's what I usually gun for:

Meta-gem:  [41380]
Blue gems:  [40119]  (and its lesser-quality variants)
Red gems:  [40138] (and its lesser-quality variants)    OR   [40141]  (and its lesser-qualiy variants)
Yellow gems:  [40167]  (and its lesser-quality variants)   OR   [40166]  (and its lesser-quality variants)

Head enchant:  [50369]
Shoulder enchant:  [50337]
Back enchant:  [44457]
Chest enchant:  [44465]
Wrist enchant:  [44947]
Hands enchant:  [38990]
Legs enchant:  [38373]
Feet enchant:  [39006]
Weapon runeforge:  Rune of the Stoneskin Gargoyle OR Rune of the Nerubian Carapace (x2)

2)  Basic attack rotations

First, some abbreviations:

IT = Icy Touch
PS = Plague Strike
BS = Blood Strike
HS = Heart Strike
DS = Death Strike
DC = Death Coil
OB = Obliterate
HB = Howling Blast
FS = Frost Strike
SS = Scourge Strike
DnD = Death and Decay
Pest = Pestilence
BB = Blood Boil
HoW = Horn of Winter

Basic Blood dps/single-target rotation:
IT -> PS -> HS -> HS -> DS -> HoW -> DC spam
DS -> HS -> HS -> HS -> HS -> DC spam

Basic Blood multi-target tank rotation:
DnD -> IT -> PS -> Pest -> HoW -> DC spam
DS -> BB -> BB -> DS -> DC spam

Basic Frost dps/single-target rotation:
Talk to Quisjaa.  He does Frost, I don't.

Basic Frost multi-target tank rotation:
Talk to Quisjaa.

Basic Unholy dps/single-target rotation:
PS -> IT -> BS -> BS -> SS -> HoW -> DC spam
SS -> BS -> BS -> SS -> DC spam

Basic Unholy multi-target rotation:
DnD -> PS -> IT -> Pest -> HoW -> DC spam*
SS -> BS -> BB -> DC spam*                                  * = only the first rotation on the pull.
PS -> IT -> BS -> Pest -> HoW -> DC spam
SS -> BB -> BB -> SS -> DC spam