Read "Eating Well for Optimum Health" by Dr. Andrew Weil.
All grains should be organic, and anything made with grains.(Flour, cereal, crackers, soy products)
All dairy products
These are just a few- read the book for more info.
A little something I learned- don't buy the junk food and it will be easier to buy good, fresh, organic produce. Provided you can get it fresh in your area. I make my own snacks from scratch with whole grain organic flour, sugar, honey, etc. It is sometimes cheaper that buying the refined boxed junk!