eating only does not help, eat a lot and exercise at least 3 hours in a week. You will gain weight and your body will be firmer as well. About eating, eat anything that you like. If you like pizza, eat two slices every day, if like eggs, also eat one boiled egg every day, if you like eathing chicken curry with rice, do it for about 3 times a week. drink a lot of water as well. that's how i gained weight those days, later i had to do more vigorous exercise and stay on diet to control the weight gain.