Mousetrap car confusion. HELP!?

That's easy. Assuming your car is built with a string connected to the rear axle pulling it when the trap is set off, just wrap it one direction, then halfway through change directions and wrap the other way. This will cause it to change direction halfway through. For the actual building of the car, just tell the people at your local hardware store what you're trying to build and they'll practically do it for you. Use balsa wood for the body of the car.