I am 21 years old. i am underweight.how to gain my weight?

See Dr., then eat...: If one is truly underweight (based on height, family background, and body type), a Dr. can evaluate for a hormonal, digestive, or other disorder. If no medical cause is found, then one can add weight by eating more calories of nutritious foods, while also doing weight-training to increase muscle. Proteins: egg white, chicken breast, salmon, ... Better fats: olive oil, peanuts, almonds, tofu, ....

Why do you ask?: You probably want to gain lean muscle. A guy doesn't really need any body fat. If you are competitive in your favorite sports, and you exercise regularly, you're fine. Elite track-and-field athletes are often "underweight" by the tables. You'd do best to do resistance training daily, different groups on different days, increase to as much weight as you can do with good form, and eat sensible.