Look at the positive side of this aspect. You look younger for your age, which is a good thing. Everyone wants to look young! If all is well, then why do you worry about putting on weight. You will gradually put on weight as you grow older. You haven't mentioned whether you are a girl or a boy? If you are a girl and you are having regular monthly periods, then there is nothing to worry about. All that you can do is to stay active and eat well because being healthy is more important than watching your weight!. At your age, you must focus on eating a healthy diet with plenty of fruits like mangoes,bananas, fresh vegetables including potatoes, cereal grains like rice, pasta, breads etc, milk and milk products etc. Sleep well and relax yourself which is very important if you want to gain weight. Cheer up!