I can honestly say there isn't anything someone can simply TELL you that will give you the answer. Someone would have to sit down with you and show you. However, there are plenty of books on sketching fashions that you can find online. If you remind me, I can give you a couple names of some that helped me a bit.
Shakeira - 90degreez has partnered up with Fashion House Academy which is for youth ages 10-20 desiring a career within the fashion industry. We are starting the pilot program this fall season in NYC but hang tight - we'll be bringing it to Philly. There will also be Fashion Boot Camp in NYC in the summer. Stay tuned for more information but in the meantime, read here what FHA is all about and be prepared: http://www.90degreez.com/page/fashion-house-academy
One way you can learn is go on google and type in croquis-this will give you the basic female shape and all you have to do is put your designs on it. There are many tutorials on the web as well that show you how to illustrate and create texture. These are the fastest and cheapest ways of designing, this will also give you a foundation to build on when you are able to learn from someone else.