I'm an aspiring fashion designer too :)
my school happens to offer fashion design as an elective, so that was lucky, but here's some ideas on how to start:
-research, research, research...find books on fashion history, fabrics and construction, fashion icons, etc.
-body proportions: find a book that teaches you how to draw fashion bodies...many books will teach you shoulder and hip angles, the general shape of a torso...that kind of thing will get you started
-inspiration: buy LOTS of magazines...cut out your favorite designs and outfits...however, you can find inspiration anywhere...art, buildings, even on the street..
-beginning design: start off designing clothes on a body stencil ( you can print them for free off the internet) and later start drawing the bodies on your own...that's the hardest part
Another tip is watching Project Runway...those designers are magnificently creative...just look at christian siriano
Good luck! and welcome to the wonderful word of fashion design!!