A good place to start is the produce aisle or even better your local farmers market. Fill your shopping cart (or shopping bag) with fruits and vegetables of all different colors. By eating foods of different colors you are getting a variety of important nutrients that your body needs to fight disease and keep you healthy.
Try to avoid most packaged foods. Most packaged foods that come with health claims and pretty, shiny wrappers are not good for our bodies and they have more of a negative impact on our environment because it takes more energy and resources to process and package items than it does to sell the whole food without processing it.
So vote with your dollar for a healthier planet and by doing so you'll enjoy the added health benefits of these fresh, nutritious foods. If you are already making most of your meals rich in produce you can focus on making more of your purchases include local and organic foods.
If you want to learn more the book by Mark Bitman called Food Matters is a great place to start.