The absolute worst thing you can do to yourself is to worry. The next worst thing you can do is starve yourself to death or fail to get proper exercise. If you look like something that just crawled out of Dachau, that has to be the ugliest and most frightening thing that exists.
Let's say you're five feet tall. Then at nineteen years old, your healthy weight is 125 lbs. Lower than that, and you are in for diabetes mellitus, early-onset osteoporosis, and assorted neuropathies. Not a good sign.
The idea is to build slowly and surely to your ideal weight.
Start with a professional trainer and a physical examination. He will doubtless tell you that you are in very severe danger of malnutrition and all the evils that it causes. He will then tell you how the vegetarian Ewell Gibbons died -- of malnutrition.
Your trainer will prescribe a very gentle remedial workout schedule -- not much more than a warm-up, 3-4 times a week. And he will prescribe a diet rich in protein, naturally occurring amino acids in your food, fiber, and complex carbohydrates.
Chemical, if you don't start taking care of yourself, look at a wheelchair sometime: if you don't get in shape and up to healthy weight, that will be your home for life.
http://en.wikipedia.org/wiki/Nutrition
