What kind of requirements do you have? Here are some points to consider:

1. Any restriction on programming language used?
2. How large/small should the program be?
3. Will the program be downloaded to a user's computer to play online? (Or do you simply load it onto a web browser to play?)
4. Is this a game you play against the computer, or against live people? Or both?
5. Do you prefer the DirectX or OpenGL API to render 3d hardware acceleration?
6. How much detail would you prefer in the scenery, cars, etc.?
7. What is the target pc/online requirement to play the game (for example, Pentium 3 at 800mhz, you need at least a 56k modem)?
8. Do you need to have a free and pay version of the game, or is this just going to be something offered up on a free gaming site like www.miniclip.com?

Just some ideas to get you started.