Well first off you’ve got to eat. Get your diet in check. You can’t expect to gain muscle mass or weight for that matter if you ain’t feeding your body with sufficient calories. All the training in the world won’t help ya unless you sort out your diet. A general guideline for “hardgainers” is to take in roughly 20 calories per pound of bodyweight. Break this down into a balance of quality protein, carbs, and essential fats and you're well on your way to making gains.