If I'm new to a dungeon, I'll pipe up right away and tell people, "Ooh, never been here before! You point, I kill!" As far as explaining boss fights, I try to be humorous. It eases people's nerves and makes the group more fun. Apart from any specialty strat, like the lanterns in Haukke Manor, I give you dungeons 101:

1) Get out of stupid. This includes anything bad dropped on the ground.
2) Hug his butt. Try to stay behind the boss, away from cleaves, breath, whatever.
3) Kill his friends. Pretty straight forward.
4) Run like hell. He's doing some AOE thing. See 1).
5) Kite his ass. Drag the boss around in a circle, he can't stay in one spot.