Yeah, if you haven't already, start lifting, and eat everything in your kitchen. I'm not kidding, eat... a lot...
Dead seriously I EAT A LOT! And lift weights regularly(3x a week) But I don't gain weight like that. I went on vacation to my friends house and all I did was eat sleep and play madden I gained about 3 lbs in 2 weeks but I was so out of shape and hadn't play ball in so long.