Eat more calories. I know it sounds easy, but it is often hard for thin people to gain weight. First, be sure you are really too thin. Some people think they are but are not. Check a bmi (body mass index) table; a healthy bmi is about 19-25. Also, consider evaluation to rule out diseases that might cause low body weight, like an overactive thyroid. Then, if still needed, work toward eating more healthy calories.