How to RP? Well this is also my first RP, so I'm not the best expert there is but I'll try.
An RP is basically a story written in the 3rd person. The only difference is that there are multipe writers and that each writer writes for his/her character(s). What follows is a story which is not set in stone from the beginning and evolves as the characters interact with each other and their environment.
To Rp, you make your character in the character thread. I suggest you check out other people's characters to get an idea of what to think of. Then you can use that character in the playground thread while the planning thread is used to plan things.
Finally, here's a small hint regarding things. If an action of your character requires a response from another character (that's not controlled by you) you should only write about the action of your character, and wait for the other rp'er to decide on the outcome. I'll take a simple combat scene for example between Bob and Ted:
(Wrong version).
Bob's writer:,, Bob leaped forward and swung down his axe. The blade struck and sank deep into Ted who dropped to the ground in pain."
Ted's writer:,,........"
(Right version).
Bob's writer:,, Bob leaped into the air and swung down his axe."
Ted's writer:,, Ted tried to avoid the blade but failed and the axe sank into him. Ted dropped to the ground in pain."
The outcome is the same, but at least Ted's writer had the chance to decide his character's fate. (In this case, it was likely an unpleasant fate.)
The best hint I can give you is to read the playground thread to get a feel of how things are done.