My favorite kit is the medic. The gun is good for close range, and the ability to heal yourself is very nice (walk thru barbwire and then heal yourself is quite cool).
But each kit has its uses, learn them and use them appropriately.
Don't camp at the planes. Practice flying and bombing on your own so you will know how to do that in a real game. I still haven't figured out bombing quite right, so it does take some practice. The planes are extremely powerful if you can nail those bomb runs.
The tanks are very good and different too. The Sherman is obviously a light tank while the Tiger is a heavy tank. That is, moves slower, heavier shell, more armor, etc... Learn how to drive, turn and shoot with accuracy. I'm slowly figuring out how to do that and its killer.
Sniping is hard for a few reasons. An upper body/head shot is required for one shot kills. And leading a moving target can be very difficult due to the netcode. Also, make sure you play on a server that doesn't zoom in on the you after you kill someone, because then they know where you sniped from and they will come after you.
And don't teamkill/tk!!!!!