Major health differences between artificial and natural foods?

So my friends and I have decided that we will try to eat natural foods and little or no artificial foods until Christmas. We’ve heard that you feel a lot better, you have more energy, etc. Umm, so I was wondering how bad artificial foods really are, and some main things to stay away from and some simple things that are healthy and inexpensive. We’re 13 and trying to develop good eating habits; we’re not anorexic or in any bad weight situations, we just want to be healthier. Thanks, any additional info would be great.