Start eating more than usual, but don't exaggerate. Eat junk food, but not to excess. Also eat more healthy foods than junk food. Try eating lots of meat, it's protein which is mass, which is weight. Try protein shakes and add kool aid or something because they don't taste so good. Exercise every single day. Drink lots of water. Don't drink a lot of soda.
Speaking from experience, just make sure that you are eating well. Vegetables, protein,(like fish and chicken), and also don't forget your daily fruits.,and eat regular meals. This advice isn't intended to make you gain weight. It may never happen! My advice is to keep yourself healthy. Be happy with the body that allows to live this wonderful adventure.