Start with your shopping trip. Don't shop when you're hungry or you'll be more likely to make impulse purchases of snacks and other fatty foods. Look at the nutrition labels on packaged foods. Many now illustrate the Food Pyramid that shows the balance of foods you should have in your diet. You can tell at a glance where that food fits.
Nutritious food can be just as quick and easy as fast food.
Fruit could be considered nature's fast food. Keep fresh fruits and vegetables handy for snacks. Prepare carrot sticks or fruit slices in advance so that when you're hungry all you have to do is grab a good snack. Make an extra portion or two when you make a meal, then freeze it. Nutritious meals like pasta and vegetables don't take long to prepare, to begin with.