First things first... dont use CoD tactics, This game is all about helping your team and going for the objective, NOT FOR KILLS. Don't sit back sniping like half of the scrubs do. Also use each class to its limit,

Engineer - repair tanks and stuff. Never hope in a tank if you're not using the engineer kit.

Assault - Always give ammo to teammates

Medic - throw down a medic kit to heal hurt teammates, revive dead teammates, only when there's no enemies around.

Recon - throw motion sensor balls where you think enemies are, use your c4 to blow up enemy tanks, by sneaking around to the tank, dont just run out in the open.

Also, if you see an enemy thats kinda far away, hit the Back button to spot him, so your whole team sees him. it helps alot.