Show up to the race when you said you’d be available, head to the XXX tent and tell anyone with an XXX logo on him or her that you are there to volunteer.  You’ll be directed to someone who will get you up to speed and get you to the spot where you’re needed. They will all be friendly and if they aren’t you can kick them in the shins.  Then, you stand in the spot assigned and make sure people and other things don’t cross the course when racers are coming.  You’ll find it to be pretty instinctual, like how you tend to not walk out in front of moving traffic.  AND you get to wear an orange jersey, like a crossing guard.