How to get rid of a double chin and get in shape?

I'm a fourteen year old girl and I'm very thin... Thing is I'm sprouting a pretty good double chin because I've been eating so much. After I noticed I was getting a little pudgy in the neck I went jogging today, and I was very sluggish and realized I was also out of shape... I want to get rid of that fat deposit and get in shape, but I really don't want to lose weight anywhere else because I'm already a bit too skinny. In fact I wouldn't mind building some muscle... So is there any way I could pack on some meat and lose my extra chin without getting any thinner?