What to do if i'm underweight and wondering how to gain weight. how can i gain weight and gain muscle?

Fitness is the key: You want to gain muscle, not fat. You know that the "ideal weight" charts are subscience, and that many elite track-and-field athletes are "underweight". Different men gain muscle with more or less ease, but all can gain some; creatine's your best supplement, eating sensibly with enough protein is key, and resistance training daily (different parts on different days) will change your body.