If you're looking at a regional theater (the kind where they actually pay people, albeit a pittance and most of the actors have day jobs), there will usually be a community theater in the area. If there's enough of an audience to support a regional theater, there will be others who want to do it just for fun.

Even if they don't have a place in the cast, they will often need techs of various kinds: lights, sound, props, costumes, stage management. It's a lot easier to cast somebody that you know can do things like "show up on time" and "be responsible". Actors... yeah. Even non-professionals.

(Bonus: I still use a light board with an actual floppy disk. The thing it replaced is literally a box of dimmer switches that they call Old Sparky. It's still in the closet. Theaters are fun places.)

Some areas really don't have community theaters. Mine had plenty, but I did actually start a theater of my own on shockingly little experience. Rent a performance venue, find a place to rehearse, get a script, put some notices on the relevant Facebook groups, and get ready for some Drama!

There is all kinds of training you can get which can help but ultimately you have to “just do it”.