quite frankly, this is a tricky time to be getting into tanking dungeons.
tanks have recently been nerfed and dps is doing massive amounts of damage. it is difficult to keep aggro on everything all the time. this is especially true if the big pewpew is just letting loose before you even have a pull organized.
while you know the game, tanking is a different mindset.
I wrote something a while back you might find helpful.
also, check out videos by quazzi, tactyks, equinox, dorki, yodatv