Eat your veggies
The term "junk food" means different things to different people.
Avoid fast food as much as possible as to me it is the quintessential junk food. Avoid excessive sugars and all foods that come in a box, as they tend to be overly processed. In general, foods that are the closest to how they are found in nature are the most healthful. Fruits, vegetables, whole grains, grass fed meats are best.