I've always loved fashion sketching & drawing up some clothing ideas, granted I wasn't very good when I started out. I'm still in the process of perfecting my fashion sketch, but I wanted to share some of my tips and tricks for learning how to perfect your sketching skills.
1. It's ok to suck. A little harsh, but you should know that everyone has to start somewhere. Your first sketch might not be a prize winner, but you have to practice, because practice does make perfect. My first few drawings were terrible, they were immediately thrown away. Don't be afraid to mess up or draw an awkward hand (we all do it), it's all a part of the learning process. Once you start to get a hang of how you like to sketch, you will find your niche and grow within it.
2. Find inspiration. I would recommend searching through some magazines or even googling croquis, which is a drawing you can trace over and over again to practice. I found mine here. Tracing either a magazine figure or a croquis will get you more comfortable with the body type you want to draw for.
3. Draw what you love. Find out what kind of style and fashion you love and draw that. Get inspired by other designs, photographs or even looking out your window. Your possibilities are limitless.
4. Find some resources. There are tons and tons of books that can help you with figuring out the pose you want to sketch. A few good ones I found are Figure Drawing for Fashion Design & Fashion Illustration: Inspiration & Technique.
Hopefully I have inspired or helped you to pick up that pen or pencil and start sketching! If you have any tips or tricks feel free to leave them in the comments!