Do a medical check up first and follow this.Eat more food.More frequently.Avoid fast foods,junk foods.Use dense fruits and vegetables,whole wheat,whole
grain extra.Take fish,egg.lean meat,cottage cheese,soy milk,soy product,mushroom,yogurt,nuts and seeds.Add healthy fat like canola,coconut,olive,refined vegetable oil in cooking.Add ghee,cream,butter,jam with food.No water before and during meals.And proper rest.These will help you to gain weight.