Well, first I would eat MORE food. Such as steak, fish, eggs, yogurt, etc. Lots of protein. Lots of calories. Look out for soda and snacks such as ice cream and candy.
There are supplements for gaining weight. Depending if you want flab or muscle, eat it and work out for muscle, or simply eat it to get heavier. I myself eat Designer Whey Protein powder. It works for me. I work out about 3-4 times a week. I used to be little, but I built up. Most importantly yocracking, eat more HEALTHY food.