Welcome! So, a brief overview of how RPs are set up here. There's Fun and Serious RP. Right now in Fun the most popular are the Snack Den and Galactic Society. Both you can hop into without having to make a character sheet, though for Galactic Society it wouldn't hurt to message the creator for a hook to get started. Serious is well... more serious. Generally people will advertise their RP, and people can sign up, usually with a character sheet. You cannot join those without prior permission. As far as I can tell right now, there are several serious RPs going on, but none are currently open to new members, though that doesn't mean you can't try and start one.
As for Do's and Don'ts, we have guides pinned. Or you can ask me, since I'm the one in charge of monitoring the RPs. In general, be nice, don't control other player's characters, and most importantly, keep things PG-13.
Once again, welcome aboard!