I find drawing from live models with cloth draped around them is the best way to begin. This helps you establish proportions as well as how to work with perspective and foreshortening.
Draw people. Carry a sketchbook and when you can draw them. Gestural sketches tend to be more helpful then long and developed poses. Copying from a magazine can also be helpful, you learn what kind of lines can make certain shapes etc.
I came from a product design background and learning to draw technically in what is called "tight line" (extremely crisp clean lines) really allowed me to draw quicker and with more precision.
I would suggest learning to draw the traditional way before attempting fashion illustration, it will make it so much easier.
another very helpful thing is to take up watercolor painting. If you can take a class on it I would suggest it. Mediums such as markers and ink/watercolor are popular (it all depends on your style) and learning how to model with pigment rather than just line or value (traditional drawing) will make fashion illustration easier. Color pencils are also quite popular but I never really figured out how to use them properly.
I would focus on:
figure drawing (from life!)
drawing clothes from magazines
clean technical drawings
and painting with watercolor and markers
what you find to be the most helpful depends on your style and you may end up discarding all of this information.
A good book is "9 Heads". check it out at the libary (it costs around 80 dollars). It's a very good and easy introduction to fashion illustration but prior art experience will make it much easier. good luck.