To start tanking it only really requires two things, know the boss and pay attention around you to possible mobs that scape your agro.

Later on you have to take care with tank buster attacks from the boss and learn tank swaping, but it's nothing complicated really.

If you are new to a dungeon, just let the party know and make small pulls, it's easy. Just act as the leader of the party, because for all effects, you are when you are tanking.