How to Target a Tank

What exactly do you mean? Well, I will count down everything I can think of:

WASD - moving the tank (W forward, S backwards, A for left turn, D for right turn)

RMB - zooming in and out

C - changing between first and third person view

Mouse movement - aiming

F1 - switch to driver seat

F2 - switch to gunner seat

And if you mean locking on to the tank... you have to make sure that your weapon locks on to ground targets (Javelin and laser guided missiles and shells will always do that; some weapons, like SRAW, will lock in if target is laser painted - you can know that something was laser painted when you see a big, red diamond with a dot). If you have confirmed at, you only have to aim down sights of your weapon. It will start beeping faster and faster as you hold the lock. Once you've aimed long enough, you will hear a long beep and then you can fire. However, if your shot is not laser guided, you have to keep on aiming at the target until your missile hits.