You need lots of carbs and protein, like Brett said. However, try not to get your carbs from Potatoes, white bread, rice, or anything sugary. These type of carbs break down quickly and won't help you. I recommend lots of wheat breads, leafy vegetables, and beans. These are typically known as "good" carbs.

You might also want to look into some sort of weight gainer powder, but be warned, these tend to have lots of sugars.