Hi and Welcome!
One of the great things about this habit ... er ... hobby is that it's highly customizable to you. You can define your collection any way you see fit and trade, or not, as much or as little as you want. If you only want to collect, great. If you want to limit your trading to cast members lanyards and pin boards, that's perfect and a great way introduction to trading pins. If at any point you decide to venture into trading with other collectors, many people would be more than happy to trade with you.
In my opinion, the best way to start collecting is to pick a character/theme, or a few characters/themes, to collect (which you already have done) and start buying pins. You can buy pins many different ways, both directly through Disney and various online resellers (including here). If you want to acquire older complete sets, I think your best bet might be either secondary retail or attempting to trade for them - although whether through sale or trade, you'll probably have to get them in pieces rather than in one go.
