It sounds like you're already plugged in to the fashion world, so I'd start there.
Demonstrate a consistent ability to deliver on time and with quality. Demonstrate a clear artistic vision. Unlike people selling portraits or wedding packages, you'll be dealing with people that know what an artistic vision is, so you damn well better have one or more handy. You'll probably also want to demonstrate an ability to duplicate other styles, as needed. Sometimes a client wants you, sometimes they want a knockoff of someone else, maybe with a little you mixed in.
You can probably figure out your own artistic vision yourself.
Copying others is about a) being able to read the lighting in a photograph quickly and easily and b) being able to guess at the post-processing.
Reading lighting is about observation. Look at the eyes, much of the light will be reflected there. Look at the backgrounds, the hair, look everywhere in the frame. Find all the lights and where they were. Estimate from the shadows how diffuse each light was. Practice, practice, practice (looking at pictures). I think there are many "strobist" web sites that have pictures together with the lighting setups. Look at the picture, guess at the lights, then look at the lighting setup. Keep at it, picture after picture. It's pretty easy to get pretty close, once you get the hang of it.
Guessing at post processing means: being able to read color balance and tonal placement, for the most part. If you can see how color and contrast are managed in post, you can get most of the way there toward duplicating a "look". Again, this is just practice. Go fool with a good photo editor. Adjust color balance, saturation, contrast. Use the curves tool. Desaturate specific color channels, not the whole thing. Learn how pictures behave under abuse.
All of this stuff will flow into you own artistic vision, and will help you clarify and define your ideas.
Oh yeah, you'll want a camera and you'll need to spend a couple hours figuring out how cameras work. They're pretty simple, though. You'll only get into trouble if you get the idea that this stuff is hard, and then, lo, it will become hard. Find a resource that makes it seem easy, and ignore the ones that complexify it for you.