Just walk into the parks, you'll see the pincarts in several spots. There are pins available in many stores as well, you can ask any CM about where you can buy them.
If you want tradables, I recommend that you go to cheappins.com and stock up on several of them for great prices. When you see a CM with a lanyard (he/she will wear the nametag as well) just walk up. I always ask:"can I see your pins?". If I do not see anything I am interested in trading, I just say thank you and keep going. It is a blast to trade with CMs, they get a kick out of it. In several pin trading stations they have a book that you can look through and swap pins out of it, by yourself.
There are some spots with tables for collectors to trade. You'll see a big "Pin Trading" sign. That's where the experience becomes more personal. While CMs won't reject your pins, not every collector will accept your trades. It depends on who you meet and what they seek. I'd be more cautious of trading with them until you are more familiar with the pins. You don't want anyone bamboozling you into losing your best pins!
What cracks me up is that with all the hoopla we see out here, many CMs did not even know where the pin stations were located in their park! You actually do not see too many collectors walking around with lanyards. In my last trip I saw, I'd say, less than 10% of the guests with lanyards. Most do not wear them, and in some parks (like MGM), you can hardly find a CM with a lanyard. So, those interested in trading gather by themselves most times.
Get what you like, and you'll enjoy your pins more. Don't play chase against others, racing to get anything that comes out and your collection will be more maningful!
Wanda