How to gain weight (in a somewhat healthy way)?

Diet and exercise. You need to eat more calories than you would normally burn, preferably making these extra calories come from proteins. Try to get between 1.5 to 2 grams of protein for each pound of body weight. Try to get as much from your diet as you can (beans and legumes are good sources), then supplement what you need with protein supplements (whey or soy protein powder).

You also want to lift weights. You should go to positive failure, between 4 to 6 reps for each set. If you can do more than 6 reps, you need to add more weight, and if you can't do at least 4, you have too much weight. Each workout, push yourself to do a little better than your last, either with 1 or 2 extra reps, or by adding 5 more pounds and getting between 4 to 6 reps.

Eat lots of lean protien. Turkey, ham, lean red meats. Also incorporate lots of weight training.

For football, I am assuming you would play defensive back or receiver because of your size, so don't try to put on weight by getting fat. Because there is nothing worse than being small and slow. But if you are able to put on 10 pounds of muscle, that would help. Just keep in mind that it takes a long time to build muscle...so don't get discouraged.

Weightgain 3000

(Beefcake! Beefcaaaaake!!!!)

Eat a jar of peanut butter.

Peanut is kind of fatty, but a lot of it is protein!

(>.<)

pum some iron you'll put on tons of muscle weight