Kinda vague, but I'll try to help you out. As you're starting out, Stam is what you want, after becoming uncritable (getting 540 defense, or close to that with some resilience).
Firstly, get to 540 Defense (you can skimp on this somewhat with Arena items & the Arena enchant, the latter is apparently used by quite a few tanks, and costs 10k honour, a real bargain). So, if you don't have the 540, try to get defense gems. The Thick cut is pure defense, Enduring is the Stam/Defense cut, Resolute is your Expertise/Defense cut (Yellow, Yellow/Blue, and Yellow/Red respectively), but really, you shouldn't need to gem too much for defense, as long as you've got the badges, and you've farmed ToC a lot (normal and heroic). Really, most gemming will probably be for stam, expertise/stam, and hit or avoidance and stam. There's some decent information about gemming in
The EJ DK FAQ, so take a look in there, and check the gear list that
Jhneoh created on the previous page for some excellent gearing choices.