As I've been building these things since I was very young, I know just about every way to screw things up. I'm gonna throw out some pro tips for beginners:
1. If you accidentally get black paint on something silver, just pretend it's a grease spot. You've just made it look more realistic.
2. If you get glue all over the windshield, just smash it with a hammer and build a "hooligans threw a cinder block at me from a bridge" diorama.
3. Related to #2, attaching the rear view mirror is impossible. Don't even try. Looking back is for losers anyway.
4. The key to a smooth shiny coat of paint is years of experience and practice, a lot of patience, the right materials, and attention to detail. Don't bother; you can't see orange peel from across the room.
5. If your model has a lot of decals, don't worry about them for now. You can sell them on eBay later on, after you've botched the paint job.
6. If you want to jazz it up a bit, consider a two-tone paint job. Then throw that idea right out the window and imagine how good your car will look in a nice solid shade of gray.
7. If a part doesn't fit, force it. If that doesn't work, start hacking bits off mercilessly until it fits. Then after you've hacked off too much, use heaps of glue to fill the gaps.
8. Make sure to drop all the small parts on the floor and lose them forever. It's not like toy cars need alternators anyway.
9.Some of the parts may be warped or bent. That's fine, just shove them together as best you can and use heaps of glue to make them stay together.
10. Most parts will have ugly mold lines and sprue marks. Ignore those! It takes way too long to sand them off. Ain't nobody got time for that.
11. Ignore all of the above, do your best, and have fun. Remember, it's just plastic!