Welcome John!
I wouldn't be overly concerned with security. If you get a lower level rivalry game (at least in my area), the school district will probably have security there for the crowds and possible conflicts between the students. If you have a concern after the game, one would probably be willing to walk you to your car. You're a step ahead of me because I still need to drop some weight.
My advice would be to listen to what the experienced officials have to say and put as much of it to use as you can. Read the rule book/case book/mechanics book about 12 times every month, take every game offered that you possibly can (never hurts to help somebody out of a bind) and attend as many clinic\camps as possible. That is what I did...got part of my schedule last night and have 4 varsity games in my second year
(not big schools, but certainly more than I expected and a lot sooner).
Oh, and have fun!