HI Velma,
There is no obligation to be staying at one of Disney's resort hotels to use
PhotoPass. Anyone is welcome to take advantage of this service.
There's really nothing you have to do in advance. When you go to WDW, you'll see photographers stationed in many areas of the parks. They're usually wearing a khaki-colored vest and will, obviously, be toting a large camera. Just ask them to take your photo. When they do so, they'll give you small card with a PhotoPass code on it. Every subsequent time you want a picture taken, hand the card to the photographer and they'll associate more pictures with this code number. (You can also use more than one card.)
When you get home, log into Disney's PhotoPass website and input the codes found on the cards you got in the parks. You can then look at your pictures and decide which, if any, of them you'd like to purchase. You can also get products like photobooks or an archival CD.
For more information about PhotoPass, you may want to head over to the independent website stitchkingdom.com and look for the "Definitive Guide to PhotoPass."
And don't forget to say CHEESE! :-)