I think this has been discussed a little somewhere here in CS. Maybe you could do a search? Got some info there.
I guess, for a start, find some models to photograph and build up a portfolio. Go shoot fashion events at shopping centres, etc, and if you appear often enough, people will recognise you. Make contacts from there, and if your shots are good enough, you might be given invites for some events. This is what a colleague of mine has done, and he now shoots quite regularly for web and print publications.
In the world of fashion, contacts are important. It's not just who you know. It's who knows you.
All the best.