Firstly, set yourself up a request thread (abbreviated req thread). List out your likes, dislikes, cravings, pairings and themes you'd be willing to try, plot ideas, what you look for in a RP partner, etc etc etc. Once that's up you can wait for replies and, in the mean time, check out the req threads of others. Once you manage to find someone willing to do a RP with you, set up a thread in the appropriate section (general, taboo, fan based, semiprivate, etc) and that's pretty much it. You and your partner will post back and forth until you desire to stop. Simple as that.
(methinks you haven't been welcomed yet if you are asking this though... also, for info on req threads, there's a
sticky post in the Introduction section of this site).
EDIT: I gave you a welcome to your intro. Hopefully my list of tips/facts and the like help you out. Good luck!