I believe one should eat the right amount of food according to what it is. For example, the Food Pyramid (remember from elementary school?) shows that you need a lot of grains, fruits and veggies in your diet. Meat, dairy, and definitely fats and sweets are not as important. But you still need some of the vitamins and minerals that meat and dairy provide, although you can get the same vitamins and minerals in fruits, veggies, and supplement tablets. Try to avoid heavily processed foods (which can be hard, believe me!)
Another good rule of thumb is eat the amount of food that best suits your metabolism. High metabolism? You need more calories and foods that digest slower in order to maintain a stable body weight (fluctuation should be about 5lbs). Low metabolism? Eat foods that digest faster and have lower calories.
Finally, the amount of exercise you get in a week. Ideally, you should be exercising for at least 30 minutes a day. But you should try to exercise 3 times a week.
This is all based on personal opinion and some of my wording may be wrong. Feel free to correct me if I am wrong!