If you suck at shooters - think Call of Duty, Team Fortress 2 and Battlefield - then you probably need help. You could ask your friends, but for a less embarrassing solution, you could try FPS Trainer.
It's a game that's designed to...teach you how to play shooter games, instructing you on the basic (and more importantly transferable) skills required to put a virtual bullet in someone's face.
There's of course a catch: while it'll be free to play, the game's creators will be charging for various tweaks and upgrades. Some of the possibilities raised include charging to "skin" the game to both feel (eg player movement speed) and appear more like a specific title (eg Halo, Modern Warfare).