Good to see there's someone else non-totally-experts here but me..
I have also just started out with a wheeled robot (as u can see at: Mechanics and construction, cheap, easy, non-advanced robot talk TOPIC
) First out u need to find out what the robot is going to do. Drive? Open things?... If Drive, u could start out by buying a EL motor (Eletric motor) of course to rotate the wheels. Then decide if u want 1 EL motor for each wheel (It will make the drift power very high, but would cost alot, it would weigh alot and u would need alot of power to give max power to all motors.
Give me some feedback on how u figured it out..