To customize one of your own cars, simply click on them, at which point you will see the Car menu, then select the Customize button. You are then taken to a new menu, allowing you to change the paint colour, add vinyls and graphics, change your car's alloys, tires, body parts, lights, suspension style, window tints, as well as a host of performance enhancements.
You can also upload some custom images to use on your car, really giving them that personal touch.
Click on your avatar to be able to customize him too! You can change a few areas to make your character distinctive if you are in someone else's Garage.