In order to tank properly you need to get defcapped. The cap is 535 defense rating for heroics and 540 for raids. There are no ifs, buts or maybes here, as a tank you're supposed to be uncrittable in PVE. All three specs are viable for tanking as a DK, but most people go for blood because it's 'the' tank spec due to all the self-sustain. It all comes down to preference and it isn't that big of a deal until the real endgame.
Gearing in WOTLK is very simple, just time consuming. Here are some easy things you can do to get gear:
*Join an ICC rep farm - there's usually no gear requirement and it will give you the best ring in the game after a few runs
*Grind TOC5/FOS/POS until you've got all the gear you want - these dungeons award ridiculously good gear compared to difficulty/gear needed. You don't have to do this as a tank if you don't have the gear to tank it.
*Reputation gear - you'll need some exalted reps to get the faction enchants so there's no reason to skip out on these. Most factions should have at least one item for you.
*Last and most importantly - grind heroics. You'll get enchanting mats (assuming there's an enchanter in the group), gold and 1 triumph per boss and another 2 triumphs and a decent lump of gold once you finish the RDF. Triumph badges will get you 4/5 T9 and almost full ilevel 245 gear, well enough to tank even ICC.
Either be a douchebag leech and queue up as a DPS until you can tank or blow all your gold on getting defcapped and then grind, grind, grind. Once you get rolling you'll get it all back within a few days.