Designing your own car online can be really exciting, being able to pick the color of the car, wheels, rims and many other features. Some websites even allow you to race people with your custom made vehicles. To find more information click here: http://www.guzer.com/games/createaride.php
The best site I found to design cars online is called Designer Space. You have cool tools such as choosing your car, making a portfolio of all your creations, and even entering contests for your designs. You can find more information here: http://www.designerspace.com/index.php
Check out http://www.designyourowncar.org/ . It has all the ways to design cars since "design" is such a loosely used word. Customizing, modeling, software, sketching, building, and playing games are all mentioned as well. Good luck.