If you want to sells apps for Windows Phone, you have to register for the Dev Center. That normally costs $US99, but until November 8, signing up costs just $US8.
Hit the signup link below for the full details. (As the site helpfully points out, if you're a student and taking part in the Dreamspark scheme, you can sign up for nothing.) The Windows Phone 8 SDK is also out today, so it's a good time to get involved.