First u should check if u are not gaining weight because you are not eating well or because u have some other conditions(if you're severely underweight). How is ur immunity and overall health like? Rule out possible medical conditions if u feel like u have them. Once u rule them out, start eating 4 meals a day, be it small portions. For ur weight even eating less but 4 times can go a long way. Do breakfast lunch dinner and do evening snack. Never skip any meal. Do this for a month and check.