When you get into a dungeon just tell the party that you've never tanked it before. Good chance if there's something you need to know, someone will tell you.

For marauder specifically, be aware that Overpower only hits enemies in a cone in front of you (the other tanks all have full circle AoEs.) If an enemy is behind you or too far off to the side then you might lose aggro on it (or never get it in the first place.) Aside from that there isn't much else to worry about right now. At lower levels you only only have a few abilities and there aren't any complicated tank mechanics. The game is pretty good at easing you into mechanics and teaching you everything as you level.