1) Find an official's association:
IHSA Officials Associations. Contact a few that you might want to join, and see what games/schools/conferences they schedule officials for, and try to join one that schedules games in your area. The association can probably also hook you up with experienced officials who can become mentors for you.
2) Attend a few high school games. When you're there, watch the officials rather then watching the game action. Check out their positioning, their mechanics, hand signals, etc. You can also do this while watching games on TV, although it's harder for televised games because you rarely see the officials.
3) get a copy of the NFHS Rulebook. An older copy (2006, 2007, or 2008) would be ok to learn most of the basics. Go through it slowly, and try to picture the situations in your head as you go through the book.