You don't want to put on fat at this stage of your life. You want to build muscle. The best way to do that is to join a gym and get a trainer to set you up with a muscle building program. After your workouts, drink a protein shake - this is important. You have a window of opportunity after exercise during which your muscles are very receptive to nutrients. And eat as much food as you need to satisfy your hunger. You will probably find that the exercise increases your appetite. Eat energy dense foods, like eggs, cheese, nuts, meat, fish etc. Fruit is also good. If you persist, you will build muscle. It will also make your blood sugar control easier.
