Are you trying to referee tournaments? Local or national? Recreational play? Does your local field require a certification?
These are the questions you need to answer. If you aren't looking to get into tournaments, there are few cert requirements, if any. Some fields endorse the PRO (Professional Referee Organization) Certification, however that does nothing to assist you with tournaments.
If you want to get into tournaments, look up your local series and send them an email asking what they require. From there you will know how to proceed. I wouldn't consider PSP or national events until you've gotten some quality field time in, at least three events if not an entire season. When you reach that point, however, www.pspevents.com/referees
will help you get your PSP certification. From there, it is up to you to contact the PSP asking for a spot on the referee list and if there is room, often times you will be added to the roster. It is quite a commitment doing national events however, so be sure you enjoy it first.