First, you need to be honest with yourself about your abilities and get an understanding of what actually goes on in design. This is a high competition field, and it requires a hard headed, skilled person who knows what they need to do and is willing to do it. Get a degree in the industry at a good school that will teach you the basics - not just how to use your creativity. You need to know how a garment will fit, what it costs to produce one, and how to design garments that will fit a wide range of people. You'll probably need to show sketches and other design elements as part of your application.
Know how to sew. If you don't know going into design school, learn. A designer who doesn't know, hands on, how a garment goes together is going to run into a whole lot of problems later on in life. Taking the time to learn to sew well isn't optional. Plus, if you can show employers a completed garment instead of just a drawing, you'll find that you do a lot better. Don't look down on sewing if you want to go into design. It's an integral part of producing successful fashions.
Don't just stick with design courses, either. You're going to need a good head for business, so be sure to get marketing and basic business information as well. Learn design related computer programs - these days, a lot of fashion designers do their work using CAD, not pencil and paper. You'll be putting together a portfolio of your designs as you learn. This will be what you show employers at the end of your schooling. Remember to get an internship - even if it's unpaid. It'll give you the hands on experience that many designers lack. A good internship could lead to a wonderful career opportunity. Make industry connections as much as you can, and don't just stick to fashion, either. Advertising and marketing connections can be a big help.
Fashion designers need to stay current on the trends, and be aware of past fashion, too. You need to become a walking encyclopedia of designs and techniques if you're going to produce something worthwhile, practical, and interesting. Understand that most designers in the US need to live in LA or New York City, and will get minimal recognition at first. Design jobs involve long hours, low pay, and little reward for the first few years. It takes a lot of work to stand out, but if you stick with it, your prospects will improve.
Take the time to decide which area of fashion you're going to target. After all, you'll be using different skills if you're mostly creating fashions for budget lines and everyday wear than you will if you're creating high end clothing or custom designs. Just because a garment is cheaper doesn't mean a lot of design work didn't go into it. In fact, sometimes more is necessary to cut manufacturing costs and keep the item in a price range that everyone is willing to purchase. Specialization tends to be important in design - just don't let it hurt your general knowledge. You might need to change areas of expertise later on in life.
Fashion design can be a great and rewarding field to be part of. It can also be a headache, and many designers feel trapped by long hours, tough requirements and heavy competition without much reward. You've got to know what to expect going into a fashion design field, and take the time to understand everything you'll need to know. Fashion design may not let you exercise your artistic abilities as much as you'd like, but if you understand the industry, it can still be a great career.