Hello,  to increase weight you need to eat every 2 - 3 hourly. You can prefer more of sprouts,  nuts, milk & its products, fish, chicken, eggs. High protein will help you put in healthy weight. Also eat fruits like Banana, chickoo, custard apple. Also It would be good if you can go to gym to tone your body which will also help you in weight gain. You can contact professional Dietician at nutritionist in your locality for the customised diet chart.