This game lets you do anything you want, so you don't have to do dungeons if you don't want. I'm sure with enough practice you could do tanking. PLD is very beefy and would probably be the easiest for you to get a hold of, as long as you have a patient group(run with your FC members ).

Just practice, more confidence in battle is good. I personally like to solo FATEs to practice my tanking survivability, it is a bit harder in the new world stuff though.