I was initated in college so I'm not sure about alumna initiation but I don't think there is anything you need to do to prepare for the initiation. I don't know if there is any sort of "new member" orientation for alumna initiates but it doesn't hurt to ask.
Request a copy of the Promise book when you talk to the president. It has everything the new members learn about the sorority in it (a really good read and a resource!). Perhaps you could attend some of the new member meetings with the collegiate members to get to know some of the chapter members and learn new things at the same time, I'm sure they would love to have you :-)
As for the actual initiation ceremony, it is beautiful and inspiring. Just pay attention to everything you see and hear.
I'm excited for you to join our sisterhood! Please let us know when your initiation date is!