How can I gain weight and add muscle mass?

Gain Muscle Mass. You are what you eat. Consume more protein and add exercise and you gain muscle. Overeat carbs and fat and don't exercise and you gain fat. You should eat 1 gram of protein for every pound of ideal body weight. For instance, if your ideal body weight should be 180 pounds, then you should eat 180 grams of protein a day...As long as you don't have any kidney disease.