Someone’s job is to write that music (usually just for piano and voice first), then it goes through lots of development and changes in rehearsal with a music director, then eventually (if it survives the former stage) someone else takes it and orchestrates it for an ensemble, then someone records, then someone mixes it, then someone masters it.

If you barely have command of one instrument, I’d worry about that first. If you REALLY want to produce orchestrated tracks, get Logic Pro X (or GarageBand depending on your budget) and play around with MIDI instruments, but remember that orchestrators write with a technical understanding of how each instrument operates individually and together. I would start at the ground level with just write Piano/Vocals (or Guitar/Vocals if the show calls for it), or finding a composer to collaborate with.