To increase your muscle mass you must increase your total calories. Of course, as you gain weight, you inevitably gain fat. The key is to minimize the amount of fat gained to retain the leanest muscle possible. There are online calculators you can use to determine what a proper total calorie amount may be for you, given the amount of exercise you do and your height and weight. It's usually though a few hundred more calories per day, although it can be an extra thousand or more.
Whilst in the gym, lifting weights, it is recommended for muscle hypertrophy that you lift in the 8-12 rep range. This is a loose rule though as you will build some degree of muscle, strength, and endurance at any rep range. For endurance, 12-20 reps, for strength, 3-5 reps are recommended, and for power 1-3 reps (explosive strength). It is good to work in mulitple rep ranges, although whichever rep range you choose, you will be best at as our bodies are great at adaptation.
