Fire only in 2 and three shot bursts unless creating suppression fire for teammates. Take a second to aim even under fire, the game rewards accuracy.
This is highly important.
If you just hold down the button and go full auto you will spray bullets everywhere, not just where you are aiming.
The guns have different recoil so some are more forgiving when going full-auto.
What I did when starting was find a ct/t gun I was good with and stick with that gun for a while. Once I was comfortable with it I went onto another gun. I still can't snipe for anything, but I dont mind being a medium to close range man.