hi
i'm new here, but i thought i'd chime in and give you a few recommendations. i'm a fashion design major at FIT, so a lot of what i do is sketching. like some other folks here have said, work on your figure drawing. it's so important to understand how the body moves in order to sketch more elongated, stylized fashion drawings. once you have the basic figure down, try stretching it out a little from 8 heads to 10 heads. also, remember that your style is your style. if you like longer legs, draw your croquis with longer legs. if you like smaller heads, do it. there really is no right or wrong as long as it looks good. just practice! it took me a while to get my sketching to where it is.
you can also use the work under technique, where you put a good croquis you've perfected under a sheet of paper and draw the design on top, so you don't get so hung up on whether or not you're figure is right (because you know it is!).
as for supplies, bienfang marker paper is amazing. it's great for rendering on. i don't use much pencil, unless it's for details and then i use prismacolor pencils. the markers i use are usually chartpak, they're wetter and richer. and for outlines i just use drawing pens... nothing special.
if you need to know anything else, feel free to ask!