How to increase my body weight?

Eat well. Body weight related to your caloric intake and activity levels. For an active person, you should aim to consume about 2, 000 calories/day for minimum. If you want to gain weight you should eat foods rich in calories such as milk and milk products like yogurt and ice cream, eggs and meats.. But at this age your goal should be to have a healthy weight not put on extra weight.
Why ? If your bmi is less than 18 , you are under weight may benefit by increasing your weight. Consume more calories than requirement, weight lifting.