Aaaah really? :D I'll help you.
Visit the Roleplay Theatre first and just have a calm look around. Are you not calm? Grab a cup of tea or your favorite beverage, take a deep breath and try again. There. Now head to the Roleplay Rules and glance at them. What they'll basically tell you is that you'll find RPs to join in the Casting subforum, and you'll write your actual RP posts in corresponding threads in the Stage subform. And if you have general questions or stuff you'd like to chat about, you can make threads in the main Theatre forum. IC = in character, OOC = out of character (meaning, writing as yourself and not your char in a particular RP).
If you go to Casting and click on a prefix with a + next to a head, you'll see RPs that should have open spots. SOmetimes GMs (Game Masters) are forgetful though, so if you don't find fun ones, you might wanna manually look at ones that have three heads on their prefix as well (should mean that it's full). OK AWESOME now sign up for a RP (by filling in the presented sign-up sheet in the opening post and posting it in the Casting thread) and try it out!