How can i gain my weight.i m 23.my weight is just 41kg only.height is 5.7inch?

Weight: Your bmi is 14.16, a bit underweight; normal is 18-25. Ideal body weight for your height would be about 66 kg. Check with doc to make sure u don't have diabetes or thyroid problem, that can affect weight. Lean proteins, good fats, low glycemic carbs (fruits/veggies/grains) combined with muscle building exercises should gradually do the trick. Many bodybuilders us whey powder shakes.