HyperFerret I'm pretty new to the game, playing a tank also. If you're a Gladiator, you basically have two options: single-target (main attack combo) or multi target (flash) tanking, while watching out for AoE's and adds. (Not sure about Marauders.) It's generally easier if you get the enemy facing away from the party so they can focus on the enemy. That target bar in the bottom left quadrant of the screen generally tells you how many enemies there are. There are some more advanced things later on but those're the fundamentals.
There's a pretty good "role" tutorial available from the Smith in, I think, all of the major city inns, which should be available around the time you start running dungeons. Definitely recommend it to new players. It will explain all the basic concepts of tanking while giving you a cool armour set for your trouble.
For specific dungeons, I'd just recommend letting players know you're new to it, and ask if there's any tricks when fighting bosses. Every group I've run with so far (now in my early 50's as a Paladin) has been accomodating, understanding, and friendly. Just be sure to speak up if you dunno what to do or are confused, 'cause everyone wants to get through it.
