I asked Ron Partridge at UTC how he got announcing. Ron is one of the most experienced announcers and IMHO, the best. He sad that way back when (95-98, I'm guessing) he used to own the sound system that was used for Rumble at the Rock, in Plymouth MA. Since he owned the equipment, he owned the mic. He eventually started giving play by play announcing. It grew from there and now he does a few regionals each year and nationals.
The moral of the story: Buy concert grade sound systems and bring them to off-season comps.
I guess I'm just reiterating what everyone else said, start at off-season comps.