Speaking from personal experience, DRK was the first Tank I played. Found it much easier to learn compared to going back and leveling WAR and PLD later on.

So yes, starting with DRK is fine. That it starts off right at level 30 means you'll have better equipment and more skills to learn the in-and-outs of basic tanking better than WAR or especially low-level PLD.

Once you've got a handle on it though, you are definitely going to want to go back and at the very least level GLD to 22 to get the "Provoke" skill. This will likely change when Stormblood comes out and implements the new Role-Based Skill Sharing, but until then...