Knowing the five major food groups and how much of each to eat every day is only part of the picture. The other part is choosing the best foods from within those food groups. That means things like choosing the leanest cuts of meat, using egg substitutes instead of whole eggs, choosing the freshest fruits and vegetables, etc.
Even with fruits and vegetables, some choices are better than others. Some fruits, such as avocados, for instance, are packed with fat and calories. It is important to check the nutritional qualities of the fruits and vegetables you buy, and not simply assume that all fruits and vegetables are equally healthy. One way to maximize nutrition while minimizing cost is to buy fruits and vegetables that are in season. Fruits and vegetables that are in season are usually quite a bit cheaper than those that must be shipped hundreds or even thousands of miles, and they are generally much fresher too. Of course, depending on where you live, there may be varieties of fruits and vegetables that are not available locally, so the northerner in search of citrus fruits will just have to watch the sales and buy accordingly.