[TIPS] Playing FPS

I always get the lowest kills and killed,for instance kills 5 and death 3. that is my average statistics in every COD games. I salute all other people out there with fascinating skills and fast ranked up. I don't know how they play it nicely, all I know if I met an enemy I just start to fire him but he's always faster than me. so I just sneaking around with my assault rifle for a very long time then I shot enemies from behind not dare enough to meet them face to face and the time is up. I'm the lowest scorer. would you give me some tips (not cheats) for playing FPS better? thank you

note: Rank 24 on COD Black Ops I, played since March 2012
Rank 43 on COD MW2, played since June 2012
Rank 40 on COD MW3, played since july 2012
Rank 10 on COD Black Ops II, played since it was released officially