What exactly is "eating healthy"?

I want a straight, completely true answer. All too often I find various sources contradicting one another, for example the movies Supersize Me and Fathead. Two guys ate essentially the same food and had drastically different results, which pisses me off because I can never find someone say ANYTHING (or at least it seems that way) definitive about what a diet should be. I'm 17 years old, 5 foot 8, and 155 pounds. I'm doing Insanity, which is a HIIT program. What should I eat?