Tanking has become very easy since ShB released.. if you've never tanked before I would suggest starting with Warrior or Paladin the low level start an fleshed out quests really hell to show the ropes on starting to tank..

Once you have a grap on the basics of tanking.. probably do some content that requires tank swaps to get use to swaps and being main an off tank in one fight.. then look at Dark Knight an Gunbreaker.

Overall Drk an War are currently built the same.. build a Guage an use a burst mode for damage.

Gun plays like a pds class an feels a bit more squishy compared to the other tanks (my current main)

Pally has a lot of utility with its magic an physical rotations an self heal.. its mostly played as offtank but can easily main tank too.

The choice really comes down to what feels good for you, but I really would recommend starting with the 2 tanks that begin at lv1