Cardio is good for you, but it's not going to be in the main focus of bulking up.
You need lots of protein and lots of calories to gain size, and an intensive muscle workout.
Just remember the basic principle. Muscle Growth = Training + Rest + Nutrition.
If you're missing any one of those, you're not going to get very far!
It's at least 75% in the diet. Muscles don't grow while you're in the gym, muscles grow when you're resting with the aid of nutrition! You work out and lift weights to rip your muscles up, then you eat to repair the muscles and make them grow bigger and stronger (hence high protein diet, as protein is essential for growth and repair).
If you don't get enough nutrition while you're working out, your body will canibalise itself and your muscles will get weaker, so diet is everything!
