Deana,
Hidden Mickey pins (originally called "Cast Lanyard" pins) are simply Disney trading pins that have one or more very small classic three circle Mickey Mouse image(s) somewhere on the pin. Sometimes the hidden Mickey icon is hard to find on the pin, but often it is very obvious. Most of these pins are only available by trading with a Cast Member who has the pin on their lanyard (most of them are not available in stores).
If you want to see examples of these pins, I suggest you go to www.pinpics.com and enter "hidden Mickey" and/or "cast lanyard" in the second page of the site's search tool. Some other good sites for info on pin trading are www.dizpins.com, wikipedia's "Disney pin trading" page, and you can click
HERE for Disney's official pin trading site.
I'm not sure what you have in mind for your "serious" pin trading, but I often suggest to friends that are starting to collect Disney pins that they figure out as quickly as possible what types of pins they want to collect, since there are thousands and thousands of different Disney pins in circulation. With so many to choose from, you could easily spend a fortune if you don't limit your search to specific types of pins.
You can focus on specific characters, pin sets (most of the hidden Mickey pins are sets of pins), themes, etc. For example, my son collects pirate pins, my oldest daughter collects pins of her favorite rides, my youngest daughter collects Tinker Bell pins. I personally have recently started to collect Donald Duck pins, since I consider myself a "Donald Duck Dad" among the "Minnie Mouse Moms" here on the Moms Panel. ;-)
Whatever types of pins you are seeking and trading, I'm sure you'll have a blast!