I can't give you specifics on how to be good at this game, but I might be able to provide you with some hints.
1) The single most important thing to learn in Multiplayer is
lag compensation. Depending on your own ping value as well as other players' ping values, you will have to fire slightly earlier to hit the same location on your opponent's screen. This takes an insane amount of practice to pull off every time, especially with single-reload weapons like the Sniper, so don't get discouraged at the very start.
2) For improving your Kill/Death ratio, learn to camp and to play defensively without blind-firing too much. Once again, I can't be specific, but try out new tactics and find something you like. (You might want to reset your current Kill/Death ratio if you care about how your profile looks.)
3) Always keep improving your aim. Start off with easy opponents in Unranked mode as if it was "target practice".
4) Ask someone to train you if they are willing to give some of their spare time. You can ask them how they do specific moves or whatnot. It's up to you if you want to rise the ranks.
And remember, have fun and good luck!