Same way you get to Carnegie Hall... Practice.
Play custom games with the bots and find your own style. You can get 1000000 different answers here, but you have to find your own way.
The obvious things are: Work on your aim. Learn the maps. Learn the objective and spawn points. Play with a team.
Everything else is subjective, and you have to figure out what works for you.
Everything he said but also try to stay off the enemy radar. Use silenced weapons, run with ghost and carry a launcher to help take out UAV's.
For me the most important thing is to be quick on the draw, since it matters less how well you aim if you are already taking shots on before your enemy. Quickdraw & Dexterity should be considered in your pick 10.