You need to know how to design, draw, sew, and construct garments. Do you have a sewing machine? I'd start learning how to use one. Learn about different fabrics and materials - there's LOTS. Take an observational drawing course and learn how to draw figures. My grandma is a seamstress so she taught me how to sew, and trust me it takes a lot of work. And learning how to sew is one thing, but most people have to use pre-made patterns. You have to learn how to make intensely detailed designs from scratch. It takes a lot of time and effort to become a good designer.
So just do your research and take classes. Thats the best I can say.