All you gotta do is find a plot that you think might "tickle your fancy" and then join up. Don't go for anything too literate, and when you send in your reserve or profile, explain to them that you are new. If they reject you thats okay, if they accept you then they will be able to give you tips.
Things to remember:
~ Always follow the Gaia ToS, nothing pisses people off more then when someone doesn't follow the rules and gets their forum banned.
~ Always use third person, unless specified.
~ Think ahead when planning your character and where you want them to go in the story. Also plan with other people about yours and their character.
~ Try to change your characters up, every once in a while. If you struggle for diversity within your characters, then use role-plays that already have specific roles assigned. So for instance the role play might be requesting the "bad-boy" where you usually play a typical "nice guy" or vice versa.
~ Ask for advice. When ever you get stuck, just ask for help. People usually don't mind giving advice to people, especially if it'll help better a role play.
I hope these tips can aid you. If you are looking for a specific genre of role-play I may be able to help you out, just PM me. If you want maybe start of with a onexone search thread. Either contact someone else or make one yourself.