Check out this link. Scroll down to First Step. There's a link with information to the various organizations.
http://www.naso.org/BeOfficial/
Don't wait until the season to start learning the rules. If you're interested in working football you will need to UNLEARN everything you see on Sundays. The rules are very different.
You can get rules books here:
http://www.nfhs.org
Good luck.