"Learning" PbP is the easiest thing there is: everything everyone's done can be read! :p
A thread labeled IC means it's the In Character thread, where the actual game goes on.
There'll also be a thread with the same name but labeled OOC. That's the Out Of Character thread and is where questions about rules, background info or general "table chatter" that doesn't belong on the IC happens.
So, just pick a game and read the IC thread to see how it's being played.
If you want the "behind the screens" look, just open the OOC thread in another tab/window and look at that too. Make sure to check the date on which things were posted, past 20 of the IC thread might align with page 41-43 on the OOC thread (or the other way around) because activity on both sides of the game isn't always the same.