If you have not tanked before, I really don't think it's a good idea for you to start off with Dark Knight.
It would be better to start with Gladiator/Paladin.

Anyways, here are the macros I use (brought over from Paladin):
1. Provoke with Power Slash then Unmend (depending on the distance between you and your target, you will use the relevant skill accordingly).
2. Hard Slash with Bloodlust and Mercy Stroke (this helps with slight mitigation and damage and also allows you to use the slots for Bloodlust and Mercy Stroke for other skills.)