Hey Ace, glad to hear from you!
You can order NFHS rule books from their web page,
http://www.nfhs.org/rules.htm. You can get a free elecgtronic NCAA book at
http://www.ncaa.org/library/rules.html
Most areas need you to be 18 in order to get "certified"
for high school & junior high games, but don't let that
stop you. I'm sure most rec leagues in your area would
be glad to have you working their games, maybe starting
with 4th & 5th graders. Not a bad way to make some
spare change. As for getting in shape, well, anything
that improves your endurance is good. Running, wind
sprints, lots of jumping jacks, squat thrusts, things along those lines will strenghten you legs & help keep you in
front of the pack. Good luck!