If you need to Gain weight, you need to eat enough. It is going to be difficult for a while, because your body isn’t used to eating much food. You might require 6 healthy meals a day along with a consistent exercise program.
Exercise
Exercise has in numerable health benefit. One of the main reasons exercising can lead to weight gain is that it promotes the growth of muscle mass. While you may burn off fat, muscle is denser than fat. Therefore, the weight you gain is in your muscles. Since muscles take up less space than fat, your extra weight doesn’t necessarily translate to larger size.
Once you start working out, you’ll probably become thinner.
Cardiovascular activities like walking, running or swimming will encourage the growth of lean toned muscles. Other activities such as weight lifting promote the growth of larger, stronger muscles that contribute to weight gain.
If you have a consistent exercise program our muscles will soon stabilize in size and after a time, become toned and you will gain weight.
Eating Habits
If you want to put on healthy weight you need to eat good protein, good carbs, healthy fats and plenty of vegetables to help provide nutrients to your body. Eating lots of nuts (high calorie, high fat) and using healthy oils in your cooking can easily help you reach your caloric goals too.
Protein
Protein helps you put on healthy body weight chicken, fish, meat, eggs, milk, almonds are few of them. protein Help in gaining healthy weight.
Carbohydrates
Rice, sweet potatoes, oatmeal are healthy carbohydrates. However, you can get away with eating less healthy things like bread, pasta, and even snack food on occasion. Every meal should have vegetables when possible so your body can properly manage all those calories.
Hydration
Water can help to take up space in the stomach, leading to a feeling of fullness and 
reducing hunger. The amount of water or food you have recently consumed or the amount of water you have shed in sweat.
Gaining weight is possible when a person works out regularly. However one cannot put on weight overnight.
