Start eating higher protein like steak,eggs,ground beef,chicken with carbohydrates like pasta potatoes rice.Also get some weight gainer shakes and mix it with a scoop of ice cream, whole cream and 2% milk before bed.If you are male you could also take a supplement called Creatine and mix it with your shakes.Eat every 2-3 hours and start lifting weights.If you are not worried about too much fat eat lots of pizza and burgers along with the high protein and carbs.If you do this you will gain weight.Oh ya and drink some beers too