You could have a Thyroid problem, Sometimes, If you weigh less that your supposed to, that means you should try to go to a doctor and ask about having a Thyroid test.... If you have an Under-active thyroid, you would be shorter and weigh less than a person with out a Thyroid problem, but IF you don't try to exercise a little bit more, and try eating more fiber and protein.
To gain weight, you first need to check on your diet. Make sure to eat balanced diet. Increase your calorie intake in all your main meals. You need also to take focus on hefty foods like breads, peanut butter, nuts and eat more proteins as well. Increase you meal portion and make sure to take at least 3 meals in a day.
If you have your thyroid tested and it does not show anything then have your hormone balance checked as well. Sometimes there is just nothing you can do. I should know on average I weigh between 95 and 105 lbs.
GNC and other fitness stores sell weight gain supplements, I eat before I go to bed ,and well I just eat alot of junk ! Although I'm a female, I have the same problem and its not my thyroid. I weighed 98 pounds, and I'm 5'7 about 7 months ago. I put on twenty pounds from doing what I just said, also I take alot of vitimans (alot!)