Some basic tips:
Shields are a thing, make sure yours is switched on when people start shooting at you.
There is no shame in running away when things get dicey.
If you aren't making a shot or successfully hiding, keep moving (preferably in an erratic manner) it's harder to hit a moving target.
It is a good idea to hide behind things (unless they are red explosive barrels).
With the exception of rocket launchers, it is better to land your shots on the enemy, not just in their general vicinity.
If possible, keep things topped up (if you see a charge point or ammo box use it, if you just had to use a battery or first aid kit try and replace it)
If you aren't in a combat zone, dealing with a target discretely may make things easier (i.e. try not to set off the alarm and try to avoid being seen to kill)
If you are in situation where violence is inevitable, shoot first (with the caveat that you have to hit the target for this to be effective)