Which type of exercises can i do to gain weight?

Lose fat/gain muscle: If you eat a well balanced diet and add weight bearing exercises/high resistance training, you can lose the fat and gain muscle/strength at the same time. Muscle is heavier than fat. Most folks are heavy because of extra fat, not extra muscle. Body builders gain wt/muscles because they eat well and lift/push heavy weights. Consult a nutritionist and a personal trainer if you need help. Good luck.