Man where to start?! Besides the basics of studying your rulebook and casebook my best suggestion? Get out on the mat! Find a wrestling club or the local high school team and go and try officiating a couple of scrimmages and practice matches. I wrestled in high school but after a twenty five year absence from the sport I jumped back in and there's nothing like getting out on the mat for the real thing to help you learn.
My other suggestion is to check out the NFHS discussion forum page along with
wrestlingref.com. No offense to officiating .com (I LIVE here for baseball and softball) but the ranks and discussions are pretty weak for wrestling. Welcome to the wonderful world of wrestling. Good Luck!