The best advice I can give you is don't worry about it, your body will develop at its own rate, and after a few years you should start putting on weight...

For now maybe try lifting some weights since you will gain weight as you gain more muscle (although I wouldn't recommend that because you might be too young to want to get into that and your body is still developing) and if that doesn't work you could go see your doctor and find out what he/she recommends. Also, as far as your eating habits go, try eating things that have lots of protein in them, as they should help you gain a little bit of weight, but as I said before your better off to just not worry about it, because it may be perfectly normal for you to weight what you do, depending on how tall you are.