Rubber band cars are powered by wrapping up a rubber band around one axle of the car, so that as the rubber band tries to unwind, it turns the axle and the wheels. What you want is the most power you can get for the longest time, and the car needs to be light but strong enough that it doesn't break as you wind up the band.
You take one end of the elastic and hook it to the nail on the axle hooked to the front two wheels, and then hook the other end to the nail on the rear axle conecting the two wheels. Then you wind the wheels until the elastic is tight.
This topic comes up commonly. Here's a link to where this same question was answered 2 years ago here:
https://www.sciencebuddies.org/mentorin ... .php?t=695&
This should help get you started. If I can find a unique solution, I'll post some more.