Hi Theresa.
I am not completely clear by what you mean when you say register on-line, so I'm going to make an assumption that you mean check-in in-line. (I figured since you have said you will be at the resort in September, you've booked your vacation).
The on-line check in process is so simple and easy to use. Here's how. Once your vacation is 10 days or less away, simply go the
Walt Disney World Web Site and click on the "My Vacations" Tab and again on the "On-line check in option.
You can make a request for particular accommodations in the process, but there are no guarantees that requests can be accommodated as they are subject to availability.
I hope that answers your question ( given my assumption ), but if not, please feel free to ask again and we'll be happy to help.
Have a magical vacation!
Carrie