There are no requirements for CAL-O (hence why it is CAL Open) so anyone can play. However, if you want to be successful, take the advice already given: practice, find a good team, practice, practice. Knowing your team mates and communication are essential. Even a team of extrodinary pub stars will lose to a seasoned team. I quit one because the leader was an obstinate dick hole. :/

You should try TWL first, to kinda of get your feet wet. I know there are always open rosters for start up teams.