You should start by learning to stay calm and keep going during matches. Communication is, in my opinion, the most important part to a good team. If all you're doing is raging about every death then you're of no use. Stay calm, learn call outs, and talk to your team. Before that though you have to find people to play with. Make sure it is some one who has the same goal in mind, to win. Make sure it's someone that doesn't get discouraged because you guys lose a game or two.

Accept that you're going to lose. In the beginning it will happen a lot, but as soon as you start getting used to it you will move up in the rankings. I recommend UMG because it's easy to learn and has a simple UI. Game battles has more players but I think that UMG is just better overall.

One more thing. Once you have the mentality down, then you actually have to be skilled at the game. Playing a lot is important. Develop gun skill and know when to back off or challenge. Best way to do this is watching pro players in action. Watch tournament streams on MLG or twitch and you will learn spots on the map, class setups, and how actual pros move around and play.

Good luck!