Eat lots or protein (around 100 Grams or more a day) and do weight training 2 to 3 days a week. A good protein source is any brand whey protein. Make a smoothie with milk, peanut butter, banana, strawberries or what ever you like with the whey protein and consume one in the morning and one at night. Good luck! Also make sure youre eating lots of lean meats suck as turkey and chicken.