I've done a few of these in the past. It helps if your artwork is vector-based. It's useful to try and find a profile picture of the vehicle you're wrapping to, and then you can work from there. I know there sites out there which sell libraries of cars, so if you're going to do it regularly, then that might be a worthwhile consideration. However, if you're just looking to do it infrequently, you could load up one of their images on your screen and capture a screenshot, and use that as a basis from which to design your wrap. Any printer worth their salt should be able to take your design and apply it.
If your using bitmap images, then the obvious things apply (ensure your resolution is high enough etc). Allow some generous bleed to give them something to work with too!