It can be hard to know what to eat if you want to bulk up. The main thing is to stay away from saturated fats (processed food, burgers, sweets, chocolate, etc.). Replace these with fruit and nuts.
Also remember you need to be eating more protein (eggs, meat, peanut butter) and good carbs (whole wheat pasta, bread, rice).
Aim for 3 big meals a day and 3 small ones.
Try these Bulk Up Recipes to gain the extra calories, found in Men's Health Magazine.