I have picked up a few from my old sixth forms and schools, I have reached out and said I'm available for anything, they always need people, maybe that's a good place to start.
It's definitely a challenge! Some easier than others and more enjoyable, loved doing Our House and We Will Rock You, not so much FAME (crazy!) or High School Musical (cheesy). It is important to keep your mind flexible and open to all of the different pieces, as each musical will inevitably encompass and cover a range of styles, tempos, time signatures, keys etc, so even if you do not like the song, you have to give it your all, or (in my experience) the whole piece falls flat. Another key thing is being on the ball all the way through. Even if you know the show well and know you are not in for another 3 numbers, do not drop the ball. Play every song precisely, and like your it's your last.
If you get lost, drop out, keep counting, and come back in. Basic advice that applies to any gig but in theatre this is very important, it's better to drop out than to guess and get it wrong.
Being comfortable with the pieces is important, the longer you have to read and practice the better, however this isn't always possible and there is often never any backing tracks that match so in that case your sight reading needs ot be on the ball.
It's a great experience, getting thrown in to Our House by Madness aged 16, not being able to sight-read or understand basic music theory was a challenge but one I'm glad I took on, what a buzz!
Best of luck!