You have 411 credits right now. If you look to the side where your avatar is being shown, it says it at the bottom. If you click the shop button at the top of the screen, next to chat and members, it should also be at the bottom of the list that appears upon clicking. If you click the credits shown, you will get a popup showing how much credits everyone has.
From what I've seen, I think we gain credits with every post we make. We can also steal, do the lottery, or insert real money and turn it into credits. Or force each other to read our charges.