They will send you a letter in the mail asking if you'd like to participate, then when you link your reservation to My Experience you get to customize your band. After you do that they ship the band(s) to you.
If you go on the Disney website create an account on My Disney Experience. You can set up fast passes as well as other things. You can do it up to 30 days prior to your trip. I received my bands 2 days later.
We are 45 days out from out from our trip and setup Fast Pass+ today and am hoping that we'll be selected for Magic Bands... Does it matter where we are staying since they are supposed to be resort-wide by the end of 2013?
I guess the first question to ask the OP: Are you going to be staying on-site at a Disney resort?
If yes, then you'll eventually be able to customize your Magic Band as the others have said.
If no, then you won't get a Band. (Though rumors are that they'll eventually be for sale for non-Disney-hotel guests)
While tooling around on the MDE site last night, I clicked on the link for Magic Bands and was pleasantly surprised to see that we got the bands (the irony was that I was talking to an agent on the phone and asked him if he could guarantee that my family would get the bands and as he was answering, I clicked on the link and BAM we got them - he slyly said that my profile said we would get them but couldn't tell me over the phone). I'm not sure if it mattered that we had just paid off the balance of our trip or not but we customized all of our bands last night and can't wait for them to arrive in the mail!
From being on this site I was up to date on everything that was goin on. I wa able to do FP+ and magic bands without any kind of notification at all. The day my magic bands arrived at my home, tht very same day I recieved an email and real mail telling me I had been chosen to participate...