Hello Cindy!
I love using the PhotoPass on my Disney vacations! Usually I'm left out because I'm always taking the pictures, but the PhotoPass makes it possible to get some really great family shots.
You're correct that there is nothing you have to do before arriving. When you come across your first photographer, they'll snap your photo and give you a card that will get scanned each time you have your picture taken.
Getting your picture taken is free, however depending on how many prints you would like it can get expensive when it comes time to purchase your photos. That is why a lot of people like to pre-order the PhotoPass CD. The CD will give you access to every single picture you had taken and you can also add character signatures, borders and icons to the photos. You can purchase the CD after your vacation, but by pre-ordering it you get a discount. Also, if you pre-order and decide you don't want the CD after returning home, you can get a full refund. For this reason, I think it makes sense to order the CD beforehand. The CD will give you copyrights to all of the photos and you can get them printed wherever you like.
You can get more information on pre-ordering the CD
HERE.