To join a game all you need to do is create a character. You make a name for your character, give it a back story, age and maybe some weapons (if the RP asks for it). When you've done that your in the RP.
When the game starts your free to do anything that your character can do. You can talk to the other players characters via your character, fight other characters, jump, run. press buttons, examine things whatever. A good GM (game master) or DM (dungeon master) if you're taking part in dungeon crawler will progress the story by giving you things to do, monsters to fight and NPC's (non player controlled characters [I think]) to talk to (or fight).
To sum it up, you do things as your character with other people doing things as their character.
I advise starting off in someone else's game before making your own. Get some RPing experience first.
Some Guidelines are: be polite, only do things your character would do, don't make your character too cool/badass or un-killable because people don't like that, be patient and don't make hundreds of OOC (out of context) posts, don't act like the game is about you (your one of a dozen players) and make sure you keep active in the game (certain RPs have failed because people stopped turning up).
Oh and have fun, I suppose that's important.
