I would start out as medic. One of the most difficult parts when you first start playing tf2 is learning the maps. As a medic, you can follow other people who know the maps, and observe how other people play the game. Also, you can almost never have too many medics on a team, and you will be valued even if you're a bit of a newb.
I personally found that the 2Fort Instant Respawn server (208.43.203.93:27015) was a good one to start with. First, its an easy map to learn. Second, you don't have to spend all of your time waiting to respawn (assuming you'll die a lot in your first weeks).
Good luck, and I hope you enjoy the game!