If you're talking about pre-50 story-line dungeons, all of the lower level dungeons will lvl sync you to the maximum level for the dungeon. This will also sync your gear to white quality for that level (I believe).

And as a tank, once you know how to hold agro, you know how to hold agro for any group of mobs. But you're going to run into problems if you don't know specific boss mechanics, they are as important if not more, than just keeping the boss off your munchable teammates.

When in doubt, speak up. Tell your teammates that you don't really know the mechanics of the dungeons and boss fights, chances are you'll get someone who will explain it to you.