Let me try to help you!  I'm not always the best at explaining things, so let me know if something I said doesn't make sense to you and I will try to re-explain it.

One person will start the RP (short for Role Play) with a teaser/explaination of what the RP is going to be about.  They will provide a character (or charrie) sheet for you to fill out with your own invented character.

Most of the time the RP starter will post the first part of the story from their character's point of view.  Then you and anyone else in the RP will continue the story from your character's point of view.  The RP starter generally has a rough idea of how the story will go and they will guide the story through their posts from their character's point of view.  The rest of the RPers will post what happens next after the latest post (made by anyone after the first post).

There are general rules for the RPs and sometimes the RP starter will make their own rules.  Try to follow the rules, it usually makes for a smoother running RP.  The Ultimate Guide (link below) should explain these rules.

http://www.cricketmagkids.com/chatterbox/chirpatcricket/node/443573

Hopefully this helps you!  Tell me if you have anymore questions!