It's easier to understand food choices if you have an understanding of the 3 main components--protein, fat, and carbs. These are the macronutrients. (There are also micronutrients, and the are very important too--but that is a subject for another time). When looking at food, it is useful to break it down into these parts, determine the ratio you need, and figure out how to get each from the best possible source.
Protein is necessary. You can't live without it. Your level of activity will determine how much you need, but you need it. Best source is whey. Worse is probably lunch meats.
Fat is also necessary, which some may be surprised to hear. Good fats include nuts and dairy. Bad fats are really, really bad, and unfortunately present in most convenience foods. The worst offender is transformed fats. Transforming liquid soybean oil into margarine, for example, requires the industrial process known as "partially hydrogenated". Anytime you see a variance of this word on the package, throw it away. This stuff will kill you.
Ok, that leave carbs. What are they? Basically, if it is composed of carbon and water, it is a carbohydrate.
Not all carbs are bad for you--it's just that overindulgence in the very simple ones creates serious problems. Beans, cauliflower, broccoli? great. Zucchini, eggplant? wonderful. You get where I'm going here. The more complex the structure of the sugars*, the better. Simple sugars are usually white. Complex carbs are usually colorful. Simple sugars are usually soft and sweet and easy to like. Complex carbs may take a bit of preparation to enjoy.
So there is your primer. You want to get high quality protein, good fats, and limit your intake of simple carbs.
If so, I could go into some detail on another thread.
*Actually, the proper word here is saccharide. But for our purposes it doesn't really matter.