You can make your own thread, by going to the roleplaying forum and hitting the button that says "Post a New Topic" or to join one of the ones already going, you should post in the thread's discussion thread that you'd like to join. The creator of the thread and the other people will decide if you can.
If you are intending to roleplay then good for you. We're trying to get the roleplaying forum up and running again around here and new faces are always welcome. I've written a 'Roleplaying Ettiquette and Tips' thread which would be advisable to read. It contains useful information and should prove enlightening to you. If you want to create your own thread then go ahead. It's up to you how you want to RP so if you have any particular designs or methods then you might wish to create a discussion thread to go with the topic where you can outline your thread rules. Oh, and welcome to the House of Lords RMV :wavey: And have fun.