I keep dying

Dear Samaritans, I keep comitting suicide.
Heres me shooting others with all kinds of weapons , the enemy take no damage then kill me with a single burst. What am I doing wrong 3 kills and 12 deaths per round is a bad average. Any tips , on what type of player to be and weapons to use or do I just need to rank up and get better weapons and more expierience.?
I only kept COD 4 for a few weeks as I was almost the worst, would like to get a bit better at this one. When I play bad company or Far Cry 2, i'm not the worst (about half way down the points list most times, so I know i am not totaly useless, just not good at this game yet so Help!
From a frustrated gamer.