Pin trading at WDW is a ton of fun. My two oldest girls got hooked on Disney pins in 2005 and you wouldn't believe how their collections have grown since then! Hopefully I can offer some tips that will get you started.
The first thing you'll want to do is take a look at this handy dandy guide to
pin trading etiquette.
Once you arrive (or even before you get there) you'll want to invest in a lanyard. These are readily available in the gift shops at WDW and on auction sites on the internet. You'll also need a collection of pins to begin with, preferably ones you don't mind parting with. Starter sets can be purchased in the parks, resort hotels, and at Downtown Disney.
Once you arrive for your vacation you'll notice that most Cast Members have a collection of pins that they either wear on a lanyard, or on a piece of felt at their waistline. To initiate a trade, you simply approach a Cast Member and ask to see his or her pins. In my experience, Cast Members are always very happy to show you a pin collection and equally as happy to trade. If you spot a pin you'd like to trade, just ask. You then give one of your pins in exchange for the one you've chosen. The guide I directed you to above details some of the more specific guidelines but I think I've explained the gist of it.
Pin trading is a great activity to enhance your Disney experience. Better yet though? The pins you collect will serve as a meaningful reminder of your magical vacation!