Assuming you have the controls set up, (if not that's the first thing to look into) you can control the vehicle as Shane describes, to control the turret you need to be in the gunners position. By default most ground vehicles have three positions, driver, gunner, and commander which you can access by one, two, and three (respectively) on the keyboard.
Once your vehicle is moving you can release the forward pressure on the joystick and it will keep on it's current pace unless something happens to slow you down, while in the gunner or commanders position you can effect turns by using the keys or device that operate the "rudder" axis in airplanes.
