If you are planning on starting an RP, please follow this link.
If you are planning on joining a currently running RP, most likely you will have post a character sheet (make sure you know the format) in the RP's appropriate thread. If you are confused about how to join a certain RP, try PMing the person running it.
Welcome to KMC
As KingDubya said, reading the intro thread and rules are always a must before starting. Also you need permission to post threads in this section of the forums, the link provided above is where you post your story/roleplay idea for consideration, usually determined by how many people you find interested.
This thread will most likely be closed, to prevent clogging the flow of this board.
I hope you enjoy your time in KMC, welcome and have fun ^^
-Aliies