Your weight gain depends on various factors like dietary intake, genetic makeup, hormonal regulation, your metabolic rate, your lifestyle etc. To gain weight your caloric intake should be more than caloric expenditure. The excess calories consumed usually get stored in the form of fat, thus leading to weight gain. To add muscles you need to gain lean body mass. Increase in lean body mass will also help you to gain weight in a healthy way. Strength training leads to hypertrophy of muscles i.e. increases the muscle mass. Strength training can be done with the help of weights, resistance band or tubes, circuit training etc. Along with strength training maintaining proper diet is very important as strength training increases the metabolic rate thereby increasing caloric consumption. Also the high protein diet is important as muscles are nothing but proteins.