If you live close enough to a metropolitan area where many boutiques are plentiful, or where major fashion magazines are present, you can usually apply for an internship position. Interning in the fashion industry is a great way to get your foot in the door while also making your name known with people who can possibly make things happen for you later in life.

While at 15, you might be a little too young for an internship at a magazine, you can most certainly offer apprenticeship/internship at an independent designer's boutique. He/She will show you the ropes of the industry and give you a good feel on whether or not the industry is for you. I worked at a bridal boutique for a few summers and learned a great deal about clothing aesthetics.

Hope this helped!