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Read the FAQs (top right of you page), then visit Community Chat, Players Wanted and GMs Wanted to get involved once you're familiar with the site rules.
If you want to join a game, Players Wanted is the place to be, if you want to run a game, it's also the place to put your ad. If you're not sure what you want to run and you're looking for requests, or you can't find a game you like and want a GM to run something specific for you, then GMs Wanted is the place to be.
You can also let people know in this thread what types of game or genre you're interested in. You can't advertize or request games in this thread, but you might make a contact with a kindred spirit, and that's one potential player right there.