Well generally a good way is to find a game that you like, then expresss interest in playing to the person(s) hosting the game.
Assuming you mean one of the boards roleplaying sessions, then I suggest you private message whoever is running it and ask them if you can join and all the details about how they're playing.
Or if you're a little more confident you can probably find some of the open games where you're invited to simply dive right in and enjoy provided you follow the guidelines (usually found in an opening post).
From there on... just follow everyone's lead and post your part of the story as appropriate.
