How to gain weight? Anyone can answer me please?

Well muscle weighs 2.94 times as much as fat, so do some heavy workouts like bench pressing, but make sure you eat lots of fatty foods too. As you work out, your body burns off fat to create muscle, so without fat to burn, you would slow muscle development. So eat a well balanced diet, including fat, and do lots of vigorous workouts (not like jogging or something, but things that involve heavy weights to encourage muscle development).