Want to gain weight i am 21male , weight-55kg. height-5'7 ft please reply?

Eat more: Keep track of your calories. You must take in more calories then you burn. There is app for phones called myfitnesspal which can help you keep track of calories. Eat 6 smaller meals a day rather then 2-3 big meals. Have breakfast every morning. If you want healthy weight avoid fatty foods and try to eat healthy. Keep track of calories in and calories out.