Chris,
As stated, you will need to register with the AIA in order to umpire Varsity/JV Baseball. Freshman Ball is handled by another association, so those games are scheduled separately.
As far as other levels, here is a link to the Camelback Umpire's Assoc, which schedules Men's League (NABA, etc.) games:
http://www.camelbackumpires.com/
Local Little Leagues schedule umpires either as volunteers or paid, depending on how they are set up. All District LL umpires are volunteers...
Any other questions???
Jim
PS You will need to drink plenty of water, but make sure you get some electrolytes, too! (Gatorade, Powerade, etc...)