How to gain muscle mass?

Good afternoon! I am 13 years old, my BMI is 19.8, football game, and I need to know if I should gain more muscle mass? Because when I play I find myself half without body to play some play, if I should gain more mass you end up for me the use of supplement ?. If not, some income from the use of food. Thank you! I await the answer!
Expert answer:

There is only one way to gain muscle mass: physical exercise. Food supplements help, but only when associated with physical exercise, however a good diet rich in fruits, vegetables, meats, eggs and milk associated with physical exercises have the same effect as food supplements.