I think its wonderful that you are ready to be involved with your 9th grade science projects already! I built a mouse trap car in 9th grade as well and remember that project being a lot of fun.

As for your question, there is no one correct solution to building a mousetrap powered car so feel free to experiment with different designs. Some concepts to keep in mind are friction, force, mechanics, and motion. (My first trial resulted in my car going backwards!) A popular design is a three wheel car --two back wheels and one in front. Another tip is that often with these cars you have to compromise between speed and distance. For example, a car with large wheels may go farther but slower and a car with small wheels may go faster but not as far. My main suggestion is just give it a shot and remember. Have fun!