Oh, and with regards to what Prudentia said about it's who you know, I have to say i disagree. It's extremely competitive in this profession and it really is all about looks.
If you have a particular, appealing or fashionable look, you'll get lots of casting offers.
Basically, the company who wants the advert/fashion show, poster, etc. will discuss what type of look they're after to suit the nature of the ad. If you have that particular look, you'll be asked to attend a casting which is usually with a number of other models, all wirth the same kind of look.
If the photographer and the company like you, they'll pick you. It's as simple as that.
There's a lot of money to be made in this game but it's harsh and you have to be mentally strong enough to deal with A LOT of rejection and PHENOMENAL bitchiness and catfights backstage. I saw a model practically rip a dress from another model when we were changing for a fashion show before.
Oh, and your shelf life usually deteriorates at about 25. I was spotted when I was 15 and I'm still doing it now at 33 (I can get away with looking 22) so I'm one of the few lucky ones. Once I start looking my age though, I'll be kicked straight off the books. The younger you are, the more wear they get out of you unless you make it BIG like Kate Moss.
I would definitely only stick with big agencies. Elite Premier, Models One and Storm are the best and hold all the supermodels.
Once again, good luck. X