I can't give you ALL the info, but I'll tell you what I know.
Most cast members at WDW have a lanyard or a pocket purse with different themed Disney pins on them. Once you have bought some DISNEY pins, you can trade with them. All you have to do is ask to look at their pins, and if you see one you like, you give them one of yours in trade for theirs.
Now, where to get your DISNEY pins? Some people get them on ebay and there are other "unofficial" sites where you can purchase them. We wanted to be sure we had genuine DISNEY pins, so I went to disneystore.com and purchased a set of 7 baby princess pins. This is what I gave to my SON to trade. I knew he wouldn't want to come home with any princesses, and he was fine with having these to start off. He enjoyed it so much, we bought another set while at the World.
You can also buy "starter sets" at the disneystore.com site that include the lanyard. We used an ordinary lanyard we had, but found it was a little smaller than the lanyards Disney sells. It still worked, but the pins "lay" better on the wider lanyards.
Have fun shopping and then trading!