[SoftTalker]

Basically don't eat food that comes in a box.

Eat fresh or frozen meats, vegetables, fruits, some whole grains.

In most supermarkets, shop around the perimeter and avoid everything in the center aisles.

[chiefalchemist]

Another way I've heard it stated: "single ingredient foods." And as you prepare them, try to keep them that way.

For example, adding a ton of butter to potatoes makes them 10x less healthy. More or less, calorically they become fries. Fries taste great but they should be an occasional treat, not a staple.