A great way to determine what you need to know in order to be a fashion designer is to look into what the job requires. I recommend the U.S. Department of Labor's website. Here are a few links regarding fashion designing and it's distinction from pattern making, tailoring, dressmaking. You'll find a wealth of information here to discover where you may like to work in fashion and what you will need to know.
From there, maybe you can reveal courses at your high school that are matching the job; for example, studio art with live models or rendering fabrics, fashion construction, color, etc. If the classes are not directly about fashion, you can always find ways to integrate fashion into your class projects. This will really stand out for your art portfolio to fashion colleges.
Beyond that, at home you can always play with fashion. If you love sketching, research different fashion croquis; there are several different methods: American, European. Or try your hand at creating theme boards to reveal your unique identity as a fashion artist. For styling, you can go for styling your friends for different occasions and document it with a blog. If you like math and bringing garments to realization from a pattern, you can try playing around with measurements to adjust already made patterns: bust, waist, hips. And if you are truly adventurous, as most designers tend to be, you can try draping and pining on yourself or friends to create something from scratch; don't forget to keep your eyes on lines and shadows.
Better yet, you can always take a few design prep classes. This is usually a great way to get a feel for your orientation in designing and will make your collegiate experience much more rewarding.
The best is to show your vocation for design. If you express your passion and show an interest in reaching out for it, it will show. This is something that can't be taught in design college.
Fashion Designers: http://stats.bls.gov/oco/ocos291.htm
Apparel workers, Tailors, Dressmakers, Custom Sewers: http://www.bls.gov/oco/ocos233.htm
Fabric and Apparel Patternmakers: http://www.bls.gov/oes/2008/may/oes516092.htm