Being able to express yourself on paper explains your vision to others. Whether it'll be a pattern maker or a client, or maybe just for yourself. To me, this is an essential part of Fashion Design, it is about how an idea changes from 2-D on paper to 3-D when the garment is sewn, and then back to 2-D when the Illustrator makes the garment come alive on paper.
When I was at Fashion Design School, I learned to draw from scratch, the body was divided into 8 parts and then you just filled in the garment you wanted to create. Today that is not even necessary, there are templates available that make a croquis (template) very easy. Homer Designs has a nice one, available at around twenty dollars or so.
Get a croquis, a good sharp pencil, a H or HB, and helpful but not entirely necessary, a B pencil for some shading. The get some watercolor or gouache, or colored pencils/markers and start drawing. And if you really like doing this, it never hurts to take a drawing class on-line or at your local community college.