Regarding LOOKING for a gig, two tried & true places would be on the bulletin boards in your local music store(s) and at a local college/university, if they have a music department. You might also want to check with the director at your school and/or other surrounding schools. Even if those directors don't have need of you in a school production, every school music director I've ever met has some other thing on the side and he might need you for that.
Around "big" holidays, like Christmas and Easter, a lot of churches put on productions. You might want to find some of the bigger ones in your area, then go and ask. Also keep checking the "Religion" section in your newspaper. Some churches advertise there for musicians there.
As far as advertising yourself, take a look at the above notes. Give people business cards and post flyers on those bulletin boards. I'd also recommend a professional-looking website with some music samples.
Also take a drive/ride around. Near where I live are a couple dance studios and a couple community playhouses. These folks might want musicians.
As far as preparing, start practicing a lot and get better at sight-reading, because you might have one or two rehearsals until showtime -- if any rehearsals at all. I've not checked if you're a multi-instrumentalist, but it's a good thing if you are, start a bit inside your field: play clarinet, sax and flute if you're a single reed player ... then expand to some double reeds. Learn to do some vocals and basic piano, too.
If you happen to own "lesser played" instruments, like a baritone sax or a contrabass clarinet, make sure you mention that to the folks you "network" with. That'll either make you or your instrument more employable.
Yes, I'm the Artist Formerly Known as Saxpics.
If you need me or just want to chat, e-mail me @ thesaxinfo-at-gmail.com.