I apologize if my qualifications aren't 100% on the mark but I've been writing for about six years now as a journalist with non-fashion mags and can offer some advice (although the previous posters have said most everything!).
Many of you are asking how to 'get started,' but I must stress that that is not the proper question. If you're passionate about your writing and fashion, you should already be started! Find some pictures, online or in magazines, and start WRITING. Whether it be for a blog, a website, or a little fashion zine you pass out to your friends, the first thing that I look for as an Editor In Chief is the ambition and motivation that demonstrates your love of the topic at hand. For example, we just hired two writers who have little to no professional experience but went around and flew themselves to a few events per year and constantly wrote about it in their blogs. If you have the will to go out and put together solid work without any pay, you WILL get noticed.
As for backgrounds and preparation, I personally took the path of Creative Writing in college. My actual English grades were mediocre and I didn't display much interest in pursuing it out of highschool but eventually I fell into the major and noticed that the concepts taught in these classes apply almost equally well for journalists! Writing poetry, prose, and a variety of other mediums forces you to work outside of the box with your subject and your vocabulary and writing style will develop magnificently.
With that said, however, I have to admit that the path to becoming a journalist seems very random from my perspective. People who work extremely hard in school and go through the usual steps (technical writing or journalism major, work at the school newspaper, yadda yadda) don't have a surefire entry-point into the field and neither do those who pursued the same path as me. In the end, it comes down to knowing the right people, being in the right place at the right time, and above all loving what you do. If you spend five years writing about fashion and constantly working to improve your style and flow, I guarantee that you'll find a company that inspires you, which will lead to bigger and better things!