I am 19 and my wight is only 50kg how can i gain it with using natural diet?

Hard to gain!: People who are trying to gain weight don't get a lot of sympathy, but it is hard! to gain a pound a week, you must eat 500 calories more than you use - each day. If that's too difficult, try a lesser amount for a slower gain. One trick that might work is eating a bowl of oatmeal with the trimmings (nuts, brown sugar, milk) each night before bed. Just try not to make the extra calories junk food.