Just keep playing, learn the maps, find which game mode suits you best, find which gun / perk setup you like most and go from there.
Something that will garauntee a kill:
RPD /w Red Dot Sight (pretty accurate, very large clip size)
3x Frags (spam these <3 )
Stopping Power (this is all round win) or Juggernaught (if you have lower reaction time this will buy you a little time)
Martyrdom (atleast if they kill you theres a chance you'll return the favour)
Now, that is a pretty lame setup and don't expect to top servers with it, but it'll get you something