whew, i could go on for hours about different things you could do to get better. the single best thing that you can do is learn to always strafe left and right when you shoot. many, MANY new and intermediate players have a really hard time tracking a moving target, especially if they are moving themselves. put on stalker and spend entire games moving the whole time, never stop or camp. even if you have to stop to reload or use a killstreak, once its done, get right back to running and gunning.
there are a bunch of things that will come just by playing for hours and hours, like better reaction time, more accurate aiming, and figuring out how many ppl you can kill in one clip of ammo before reloading.
here's some other things i won't go into detail on to save time/space but will also help you:
-knowing the map, especially big camping spots, and beginning spawns
-figuring out where enemies are based on where your own team is and enemy radar blips
-knowing which battles to fight and which to avoid (entrenched enemies, 3 or 4 vs you, etc)
-using grenades (great vs campers and known sniper positions)
-knowing when's a good time to reload and activate killstreaks
-being flexible, for example using assassin when the enemy has UAV or portable radar or packing a stinger + blind eye when there's an enemy helicopter
-turning your sound up to hear nearby footsteps
hope this helps, good luck!
