Looks pretty good for a first go... in fact, much better than my early attempts. I think I can offer a couple suggestions.

1. If the pic is low resolution, up-sample it by resizing it to 300 pixels/inch. It won't improve the actual pic, but it will make any modifications you do look a lot better in the final image (when you shrink it back down to post). It will have less of that pixelated look.

2. Use lots and lots and lots of layers. I use layers for everything. I have a layer that is just the windows, just the chrome, just the body, and just the background, etc., etc., etc. This way you can make modifications to each layer separately and make multiple copies of a layer to try different effects.

3. Avoid using the paint tools freehand. Use the selection tools to select an area and then use the paint tools (like the brush tool). Or make a completely separate layer of the selected area (new layer by copy) and then use the hue/saturation tool. This is an excellent approach for stripes (like your pinstripe).

4. I use lighting effects (from the Filters menu) for window tinting and chrome blackout. I find that it provides better results than just using the contrast/brightness adjustment.

Hope this helps.