Study everything you can about fashion design. Read fashion-design books, periodicals and other publications. Learn what the latest trends are and study successful fashion designers.
Practice, practice, practice. Spend time every day drawing the ideas that you have for clothing designs. Practice getting the ideas from your head down on paper. This way when your big break comes you'll have a portfolio of drawn designs ready to show.
Learn how to make your own patterns and how to sew. Being able to make your own patterns and to sew is going to be crucial to your being able to show how your designs look on a real person rather than just on paper.
Put together your portfolio. This is one of the most important steps to becoming a fashion designer without a degree because you'll be competing with those who have degrees. Purchase a nice portfolio folder and then arrange your best drawings in it.
Attend as many fashion shows and trade events as you can. Wear your own designs if possible when you go. Network with other fashion designers, models and others who are in this field.