There are a couple of options for you to consider.
The first is to study fashion merchandising in college. Many colleges offer programs in fashion merchandising. If you go this route I also recommend you participate in an internship while in school since this will provide you with some experience and further insight into fashion merchandising.
The other option is to work in a retail company that promote from within to fashion merchandising positions. You can then get an entry level position and work your way into a fashion merchandising position. Nordstrom is a company that offers this career path.