Start by trying to identify some foods that are clearly good for you and increase how much you eat them. EX, oatmeal - fantastic food, almost universally agreed to be healthy, and has no real potential downsides. Then find foods that are clearly bad for you and decrease how much you eat them. EX, Coca-Cola - pretty much no micronutritional value, all you're getting is sugars and bad stuff.

That's the way forwards. There's some disagreement on certain places, but definitely increase clearly good foods while reducing clearly bad foods, and that'll be a good start.