I'm just beginning to learn too, so I may be wrong. But I think that when you're using physics you need to work with forces to move things around. I don't know how's your model, but depending on it you could apply a torque in the wheels or a simple force, pointing forward, in the gravity center of the whole object.

[edit: actually, I don't think it matters if the force is in the gc]
[edit2: actually, I don't think you can choose where to apply the force on an object! ]