Guys, why am I so ass in this game, help me!
Start on low pop. servers, the more enemies there are the easier it is to get caught in the mess and gain no experience at all. If you start on a 4v4 payload or something like that you're guaranteed to make an impact.
Try using Heavy first, you can get a decent amount of time alive due to Medics healing you and your main weapon is full auto meaning you can spend as much time as possible getting used to the maps and a general feel for the game, rather than worrying about aiming with your grenade launcher or w/e...
Always get high ground. Even direct combat classes like Demoknight and Pyro can make use of it for an ambush. This is pretty obvious really, but when you're playing a class like Soldier height is a massive advantage.
Finally I'd say get used to the maps and learn the good flanking routes. Everyone bar the Engi and Medic can make use of flanking. Even the Heavy (who you'd expect to be at the frontlines) can make an excellent flanking class with the Tomislav, Sandvich and GRU...
TF2's a fun game anyway, as long as you're enjoying yourself you'll see yourself improving before you know it anyway :D