I was never that great at any COD games, but consistently contribute a great deal when playing BBC2.
Learning maps is actually the most time consuming aspect and just remember that gravity effects everything.
Also, the weapons you start with are not far behind those others will be using, so there isn't a big disadvantage in that department. Plus, what you lack in power from the start you can counter with smart play and strategy.
Also, leveling up isn't just based on kills, so if you help complete objectives in Rush and Conquest, you'll find yourself breezing through the first several levels in no time.