The part about getting started is easy: You pick something you want to play, hopefully with a fairly thorough knowledge of how the setting works, and hope someone jumps on it. At least, that's how some beginners think. In reality, it's more like fishing than most care to admit. The conditions have to be right, and you have to use the right bait to attract the sorts of fish that you want. The RPG Design forum can give you more ideas on how to pitch an idea, how to keep players under control, and what people like and dislike running across.
Once you have a decent group going, where you post your replies is up to the rules established by the game master. It can be in the thread (hence the term "play by post"), or it can be in the more sophisticated tab system, which can be replied to either in the tab or through its own chat room.
As for getting started as a player, it's vitally important that you pick a roleplay first and make a character that conforms to it instead of the other way around. It's okay to have favorite genres and storytelling styles, but it's good to try something new every so often as well.
Hopefully these tips helped you with what you wanted to know. Best of luck out there!
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