Hi there..First set your goal as to how much u wanna gain weight depending on your height. Foods that wil help you gain weight are- have breads made of whole wheat, oat bran, pumpernickel and rye, are more nutritious than white bread, use peanut butter, jam, honey, hummus, or cream cheese with bread, have more proteins as they help you gain weight in healthy way. Have sprouts, protein shakes, milk shakes, have vegetables like (potatoes, peas, corn, carrots, winter squash, beets), have fruits like (bananas, pears, apples, pineapple, dried fruit) , if your non vegetarian then have one egg everyday, have chicken and meat atleast twice a week. Have milk and diary products. You can have protein supplements, many protein powders are available in market, mix with milk and have them twice/thrice daily,whey protein is very good quality protein and very good for building muscle mas even so no harm having it.. do weight training exercises, gyming ( it helps your muscle mass to increase and help you gain weight and even increases your appetite) hope this helps you. Consult dietician once.