Organic foods have less toxic "Cides" in them - you know Pesti, Herbi, Fungi.  The Environmental Working Group puts out a "Clean 15" and "Dirty Dozen" list.  This helps you know what the dirtiest fruits and veggies are so you can make that your priority to be organic.  

Know your farmer.  Fresh food is best.  When you get to know your farmer they can help you get the best in season foods.  Buying from local farmers also support your community and cut down on pollutions.  

Again know your farmer.  Grass fed and  pasture raised meats and products are the best for you.