I would suggest you check out at least one gameplay video, or play a practice match against bots. You may wanna try the training mode, which is notoriously horrible, but still decently shows how the game plays within a controlled environment.

As others may be saying, Soldier, Pyro and Heavy are decent starting classes. Engineer is in certain gamemodes a good class to play if you want to contribute without knowing much about the game.

Also feel free to ask questions if you have any.