Not bad for a first submission
Here are the things I think you should try to fix/change:
1. You get 50 points if you shoot the guy that has his hands up, is this a bug?
2. Enemies spawn in a very predictable pattern, try to add a bit more variation.
3. There is one white strip on each side on screen, I assume this was a formating issue.
4. Bullet holes stay from level to level.
5. Bullet holes spawn on top of everything else (Enemies and cars). If you are creating them with the "duplicateMovieClip" fuction I know it can be problematic to get them in the right layer, but its not impossible.
6. I found the "Play Again" button hard to read, but that might just be me.