Consult handy guides, like this one [http://www.gaminggutter.com/forum/f203/newbies-guide-role-playing-20612.html] and this one [http://arkanabar.tripod.com/newguide.html]. Look at places like good_rpers_rock [http://community.livejournal.com/good_rpers_rock/] and Ursula Vernon's occasional posts about roleplaying (like this one [http://ursulav.livejournal.com/932513.html] and this one [http://ursulav.livejournal.com/935760.html]) to see how it's done. Also consult bad_rpers_suck [http://community.livejournal.com/bad_rpers_suck/] and the Mary-Sue Litmus Test [http://www.springhole.net/quizzes/marysue.htm] to make sure you don't do it wrong.
But most importantly, be humble and hopeful. You won't be able to hop in and play Xanatos Speed Chess [http://tvtropes.org/pmwiki/pmwiki.php/Main/XanatosSpeedChess] with the big boys right away, but if you learn from your mistakes, accept constructive criticism and don't lose heart, you'll come out on top and be making critical hits with the best of them.
Finally, be polite when looking for people to RP with, but more importantly, be self-aware. Learn what you like to play and look for people with similar tastes. You'll all be happier that way.
Take luck!