How do you get to Carnegie Hall? Practice.
Try a few things... I would recommend playing some Private matches against the Bots at Recruit level. Get to know the Maps, try out some setups and streaks and see what you are comfortable with. AS you improve, bump up the difficulty or switch to Squads Modes. Familarity with the maps, weapons, equipment, and streaks will make you better and better.
Also, think about what modes you are playing. Perhaps you need to start elsewhere, get comfortable with the game, and work your way to that mode.
Lastly... learn from other players, especially skilled players... but find your own play style. When you develop a comfort level with how YOU play, you will see improvement.
Just have fun with it. If you remember it is a game and about having fun, you will take some pressure off and allow yourself to play well without unrealistic expectations.