Start by finding some great photographers and build up a good portfolio. If you don't have the money for a professional, at least have a friend take some good snapshots of you. Send those to the agencies you're interested in, along with a copy of your work permit. The best way to know what each agency wants is by going to their website. Never sign up with an agency that wants money up front, rather they will profit from commissions after you are signed. And don't pay for them to hook you up with a professional photographer. Legit agencies don't do that.