One of the better ways for your Death Knight, or any other tank, to learn tanking skills is through the Proving Grounds, introduced in patch 5.4. It’s a safe place to experiment with rotations, dealing with multiple targets, reforging to get the most out of your gear, and so on. No one will know whether you took one try or 100 (or more) to get your gold achievement.
If you can get at least a silver achievement on your Death Knight (or any other character,) then you’ve probably got a clue.
In fact, Blizzard agrees that the Proving Ground is a good test and it looks like they are at least considering making the Proving Grounds a requirement for random groups in Warlords of Draenor. There were some recent interviews with Blizzard Devs on this topic.