I dont want to make 20 scenes for 20 cars, so can someone help me for a script that changes the car based on the users selection
also how can i make a left and right box selector
then i press right and changes to 
There’s a lot of ways you can do this that don’t involve multiple scenes. For example, you can activate/deactivate game objects, enable/disable renderers, move game objects around by modifying their transforms, or change assigned meshes and materials via script. Any of these methods could be used to change which car is currently displayed (or displayed most prominently) on screen.
As for the buttons, again, there are multiple solutions, but the most straightforward would probably be to use OnGUI() and GUI/GUILayout.Button().
I would need some script examples, see here : http://www.youtube.com/watch?v=lDLG7Vg9NuM&feature=related
thats what i mean for menu, + i need to make a weapons system
Maybe someone will be able to direct you to a complete example, but for something like this, I think your best bet will probably be to develop your programming skills to a point where you can at least give it a try yourself, and then post back here with any specific questions or problems that you have. (Another option would be to try the Collaboration forum and see if you can recruit or hire someone to help you.)
If you want to pursue it yourself though and need some help, perhaps you could start by identifying one aspect of the interface that you’re not sure how to implement, and then ask about that.
I’ve been thinking of this too…and I haven’t found an answer yet…
I tried to call
Object.DontDestroyOnLoad on my car,to survive the scene,but i’ve been thinking how to make the car start at X,Y,Z position in my next level.