I have some helpful tips.
1. Take your time preparing the car to be painted, and do a good job. If you don't prepare it well, it will look like crap.
2. After you've sprayed some kind of primer on it, any blemish you can see (speck in the primer, dent, etc..) will look 10x worse once the paint is on there.
3. If using a clear coat, ignore this. If not, you need to get the paint to appear "wet" but not drip. This is to achieve a glossy coat.
4. If this is your first time painting high visibility areas (like door panels, hoods, etc..) use a lighter color, dark colors show more spots where you screwed up.
5. Cheap spray guns suck, and you will get "fish eyes" (a kinda swirl effect in the paint, sometimes hard to see, happens more with metalic paint)
Thats about all I can think of right now, hope some of that helps a bit.