How do i get rid of a double chin?

im not fat anywhere except for my neck! like seriously, im a size 4! (im only 16) but whenever i tilt my head down/back, i suddenly have a double chin! and my profile view has like a saggy old lady neck! i didnt used to have it like a year ago! i just need to get rid of it ASAP! help please! no lame answers, just good tips! i'll try anything!