Depending on how much weight you want to put on, you should be eating lots of calories, 4000 good calories is probably a place to start. Make sure they're good calories though. You're wanting plenty of good protein to build up muscle mass and not fat - and that means tuna, chicken breasts and eggs.
Remember to work on alternate parts of the body, so that they have a chance to rebuild before you give them a work out (say upper body one day, lower body next, then some cardio the next, the upper again etc).