Don't run around spraying like a maniac,
don't be annoying,
use your ears,
try being quiet when your alone, (you can hear people who are running around easily that way)
make use of flash bangs/smoke grenades against campers or snipers,
don't be vengeful against people who accidently TK you,
try to coordinate with your team.. it makes the difference between a good and mediocre game.
if you wanna add me on steam, my friends name is "ReFracture" (thats not my steam id, fyi)