Just launch it and play Payload or Capture-The-Briefcase in a vanilla (no mods) server. For classes, Pyro, Medic, or Soldier would be the easiest to start with, I think. Any class would work, but those three, plus maybe Heavy, are the least reliant on advanced techniques to really succeed.
Protip: remember to attack any ally you can't walk through, because they are a spy, and that spy will kill you--plus watch out for empty corners, they could have an invisible spy hiding there--AND watch out for allies walking up behind you, because they, too, could be a spy. Really, after playing TF2 for a while, you'll develop a reflexive twitch about checking corners, behind you, and allies, as you travel through the map; in spite of this, spies will still get you. And one of those spies may be me.
Godspeed, youngling. You will need it.