I am 20 year old and my weight is only 41 kg.i am very thin, skinny man what should i do to gain weight.

Calorie dense foods: Eating calorie dense foods such as nuts, heavy cream, butter, fatty fishes, dried prunes, and avocado can help increase your daily calorie intake and assist in the desired weight gain. Supplements for weight gain would be inferior in quality to whole foods which are readily available and much less expensive. Increased protein in and weight bearing exercise to build muscle mass helps also.

Weight: Try lifting weights to add bulk and increase daily calories with good nutritional foods and protein.