I'm by no means any kind of expert for tank roles in general, but I found a thread that might be useful: http://forum.square-enix.com/ffxiv/threads/341614-An-In-Depth-look-at-Dark-Knight-Lvl-70-Raiding-Guide/

The guide in it seems to focus mostly on the high-level endgame raiding, but I imagine that looking at that and reading through the thread and seeing other players' input might give you a better idea of what you should try aiming for. Unfortunately, a lot of it is lost on me but hopefully that helps a bit. Also, I'm really sorry you got stuck with such an uncooperative group. Some people just don't have any patience for anyone who doesn't meet their own personal standards, and instead of taking time to give advice they go and act like that, and frankly it's awful. Fortunately I'm finding for every group like that there's also another group that's more supportive and will make sure they offer as much knowledge as they can and let things proceed at a reasonable pace. I hope if you decide to stick with DRK that your experience with it gets a lot better!