Gaining mass depends on a lot of factors. I'm going to assume you mean muscle mass.
1. Eat clean. Make sure you get the right balance of protein, carbs, and fat. You metabolic rate and size will determine how much you need to eat. You need to keep a journal of what you eat and estimate you gains. You will eventually find out what works for you. Also, I like the 6 meal a day thing with smaller portions per meal.
2. Weightlift for Muscle Gain. From what I've read you should increase reps (8-12 range) for whatever you can lift. You must try to lift with the burn until you can't stand it anymore. Of course this doesn't work for everyone. Right now, I'm doing a pyramid scheme until I get no more gains, then I'll change it later.
3. Cardio is good, but it may make mass gain more difficult.
That's all I know to suggest. Hope it helps.