To gain weight, it is preferable to gain more muscle mass than fat. However, a football player needs some body fat to help cushion their internal organs from tackling injuries during a game.
You didn’t say either what your weight and height were nor how much weight you want to gain so I will respond in general. If you are maintaining your weight with your current food intake, you will need to increase your calories by 500 per day to gain 1 pound per week, by 1,000 calories per day to gain 2 pounds per week. Your diet should contain less than 30% of calories from fat (16 -33 grams fat), 10% protein (13 – 25 grams protein) and 60% carbohydrates (75 -150 grams carbohydrate). Otherwise, eat a variety of foods. There is no magic muscle builder contrary to all the protein supplement hype out there. It takes daily hard exercise.
At the same time, if you are over 15 years of age, you should start lifting weights every other day. Is there a weight training room in your school or do you live near a public gym or health club? You will need some guidance from a weight training coach so that you effectively build muscle and don’t injure your muscles.
Football season is 6 months away and you can gain 24 to 48 pounds. Do it slowly with increasing your food intake and weight training so that you mostly gain muscle.