Here are some good places to start if you're looking to build one:
http://www.wikihow.com/Make-a-Soapbox-Derby-Car
http://www.ehow.com/how_2044164_construct-soap-box-derby-car.html
If you're willing to spend some money, here is a place (and yes, the kids look dorky, I know):
http://www.ehow.com/how_2044164_construct-soap-box-derby-car.html
If you are planning on using this to enter a race, it is very important to know which materials are not allowed in your soapbox car. Also, you could make a road trip to watch the Red Bull Soapbox Race and see how others have made theirs (www.redbullsoapboxusa.com). Finally, just remember to make it function well first, then add on the style.

