In our society, weight loss is all the rage. Trendy diet books, magazine articles and even TV shows devoted to losing weight dominate the media. Exposure to this information benefits many people; however, most of it isn’t appropriate for young athletes looking to grow into powerful competitors. If you cut carbs or eliminate fats, like most popular diets recommend, it will be difficult, if not impossible, to optimize your weight for superior performance.
Before embarking on a weight gain program, keep in mind that you’re gaining weight to improve performance. This goal will dictate the methods you use to gain the weight.
The worst thing you can do for weight gain is to eat everything in sight. You especially want to avoid junk food and processed foods, because they are generally high in fat and sodium. If you ignore this advice, you will likely see more growth around your gut than added muscle mass, which will be counterproductive to your goal.
Instead, you should  increase your caloric intake by eating larger portions and additional meals of healthy foods and snacks—adding about 500 to 700 calories per day with a goal of gaining one or two pounds per week. Your protein intake should also increase—to 1.5 to 2.0 grams per kilogram of bodyweight. This sets the foundation for weight gain by providing extra calories and protein to build lean body mass, instead of unsightly fat, while still supplying the required energy for powering through your workouts.
Obviously, if you increase your caloric intake without exercising, you will gain fat instead of muscle. But with continued strength training and a diet designed for gaining weight, you should pack on some lean pounds in a reasonable amount of time.
Check out this STACK article for 15 Quick Tips for Weight Gain.