Here are sometips and tricks:
1.Never camp in one place-you should be mobile.Move around to locate enemies and decrease death risk.
2.It is specially important in TE,TC,TD to flank the enemy.In gaming terms flank means to stealthily counter the enemy from their backs or sides.
3.Always check your flanks(left,right,back) before moving on since you never know who could be stalking you.
4.Burst fire means to shoot in small time intervals with gaps.This increases accuracy,reduces recoil and muzzleflash,and also saves up ammo.
5.Use the minimap to your advantage. Locate enemies,plan your routes and group up with your team.Also peek through small holes or gaps etc.
6.Shooting at the head increases damage output and doubles your exp.It also saves up ammo.
7.Never blow your cover by firing uselessly or coming out in the open.Stay hidden behind anything.
8.After clearing an area reload before you mobilize.You really don't want to end up with an empty magazine in front of an enemy spawn.
9.Take your time while aiming.Get a clear view of your target and shoot him at his head.
10.Stick with a weapon you like. switching your weapons doesnt help at all. Adapt to a gun, task it and play-with it and customise it.
11.In CQB jump,crouch,and move sideways to tackle the eniemy.While playing in medium-long range maps move sideways continuously to avoid enemy sniper's bullets.