Are you in school (middle, high)? If so get involved with the
theatre department.
Trial and error? Sure is! (We're not born knowing sound, even though others may argue that they were).
Usually though, you will have someone there to mentor you and give you enough practice time to get it right before you're actually running sound with an audience. Another way to learn without being overwhelmed is doing smaller shows first which may only have a few simple music cues and basic hanging/floor mics, before stepping up to running sound for a big musical or concert. Another way to get involved in sound in high school is to see what it would take to start helping out with band/
orchestra/choir concerts. Work with someone experienced, and they can teach you how to set up mic's and patch them to the board. Then you can learn to set levels, and run a fairly simple event. Apply that knowledge, and
gain more as you go along.
The best advice I can give you though is if you are in school, get in to
theatre. Otherwise you are not likely to touch their sound board.
If you are not in school, all info above is null and void. Instead, find a
community theatre to volunteer at. Tell them your experience
level, and just maybe they'll let you hang around with the sound tech in the booth to learn the ropes (I don't see why they wouldn't. Community theatres are usually struggling to find good people looking to learn tech). Same mentoring applies here as well. Eventually you will be good enough to do it on your own, and you can get on their volunteer
call list. You'll be running performances before you know it.
Just as a side note: You can do both. I have noticed that breaking in to tech in high school is not a simple thing to do. Some students have worked pretty hard to
gain trust and seniority in the department and will be very protective about someone new coming in and wanting to take over "their" equipment. Of course, this all depends on the nature of the people at the school. Who knows, they might be desperate for someone who wants to learn sound.