Draw live models, draw live models, draw live models.
I could not emphasize this enough. Take art classes, specifically figure drawing classes. Hone your drawing skills in general, for that matter, it's a useful skill.
If you can't find a free class, find some people with similar interest in fashion and convey to them the importance of drawing from life, draw each other. A decent book is "Drawing for the Absolute and Utter Beginner" by Claire Garcia, you'll learn basic technique on rendering what you see. And, once you advance a bit, I suggest "Dynamic Wrinkles and Drapery" by Burne Hogarth, which is about wrinkles and folds in fabric, both in general and on the human body in particular. I also understand there are books on drawing for fashion, but that's not really my area of expertize so I can't suggest a good one. You can look these up at amazon, though, or ask for them at a book store.
Definitely start studying art and textiles. Begin building up a portfolio of sketches and art work. Take some sewing and design classes outside of school in the evenings to build up some actually clothing for your portfolio. Aim for a fashion university and work very, very hard. Once you're there you'll be surrounded by specialists who've been in the business for decades and can you help you. It's also advisable to back-up your fashion courses with some form of business studies.
Take some drawing classes so you can draw your fashion ideas well, and also learn how to sew because in fashion school you are going to have to know how to make your clothes because no one is going to do it for you. Also you'll need to be making a portfolio to get into fashion school. You are a great age to get a good start on this! You totally can do it!
learn to take critisisim well, listening, education. Good luck
