how do I gain muscle mass?
Protein protein and more protein and work your stomach, arms, legs and chest. And your back as well.
To gain muscle mass, you need to use free weights for all heavy sets and exercise less often as you increase strength and size. You can do up to two days of light jogging on the treadmill, but keep it to around 30 minutes per session. You can check out http://www.mensfitness.com/training/build-muscle/10-ways-to-gain-muscle0 for more information.