1. do you have a polariser, it will help reduce glare and reflections off the car surfaces.
2. lights, if you do not have any off-camera lights, and cannot afford to get some, go to bunnings or similar and get some of those portable flood lights on stands..they work well and even if they are in the shot, they can add a grittiness to the result, especially in a garage setting. And you can use them to
light all sorts of things after the shoot is done. *shoot in
RAW so you can adjust white balance later if the lights end up creating a colour cast*
3. Prepare to do some cleaning up in the garage. You will find the results more visually pleasing if the garage is reasonably tidy, with tools away etc
4. Grab some car magazines and study the photos, the angles taken from, the backgrounds, where it looks like the lighting has been placed, etc. Try and duplicate this, after all they are in the magazines cause the photographer has worked out what works and what doesn't. So do not be afraid to copy what others have done before.
5. Magazines can also be handy to see how the model poses, and you can get your model to copy those poses as well.
Enjoy, have fun, get creative and show us the photos too when you are done!