It's actually pretty simple. It all comes down to three things.
1. The vast majority of your food should be un- or minimally processed. Meaning, the closer it is to the way it was when it came out of the ground (or off the animal), the better. For example:
- Real Cheese > Kraft Singles
- Brown Rice > Rice-a-roni
- Olive Oil, Coconut Oil, Real Butter > Margarine
- Water > Kool-aid
2. Eat a variety of foods. Try to “eat the rainbow” or get foods of all colors in your diet by the end of the day to make sure you're getting lots of different vitamins and minerals. I'm going to plug leafy greens in here because eating a handful a day makes such a huge difference, even if you don't make any other changes. Aim for plenty of protein with your meals.
3. Water. YOUR CELLS NEED IT to function properly and to be able to USE the fuel you're giving your body.
There's also something to be said for supplements, but that's not quite as straightforward. The kind of supplement you'll need to take depends on where you live, what you do, what gender you are and the way your body works. For example, I need to take Vitamin D since I live in ND where it's too cold for over half the year to go outside and get enough sunshine.
It's nothing to get fanatical about – you'll go crazy if you do. I notice that those who tend to over-complicate healthy eating are usually trying to sell sometihng.
I've gone through seasons in life where I eat nothing but crap food, and now I eat like this, so I know: if you eat this way for the most part, you'll feel a lot better in general and pretty much stop getting sick. Awesome.