Okay, to clear some things up there are 4 kinds of abilities you have as a tank. Damaging abilities, defensive/offensive cooldowns, crowd control and taunts. This is really overly general but the only thing different between tanking and dpsing is the fact that you're using your damaging abilities and taunts to make sure the big bad is looking at you. So assuming you have already been using all your CC and cooldowns the only thing different in tanking is that you've added the two taunts. Now some abilities are not very useful for tanking (an example is crushing darkness on a sin, it's terrible) but you can figure out what abilities you should be using by reading your skill tree. If anything says "adds additional threat + damage" or "this ability generates a large amount of threat" those abilities are going to be used constantly by you to keep threat on the boss for more detailed info about the abilities check out the class forums. As for the easiest tank to play IMO it's currently The power tech/vanguard, you have the most passive mitigation out of all the tanks and a few very nice very easy to understand defensive cooldowns, however assassins/shadows have a much easier time with threat (but a much more demanding rotation). And jugs/guardians are in-between good threat on demand and passive mitigation with a decently engaging rotation.
hm sorry for the rambling, all in all just throw yourself into it at low level and learn as you level up.