Royalties and performance.

Hello.

I'm wondering about the protocol surrounding using someone's play and or music.

If I wanted to put on a production or write a Cabaret using someone else's songs; how would I go about gettint permission, Do I need permission?
Do I have to pay royalties??

If someone could shed some light I'd be really grateful, I haven't been able to find anything on the subject.

Thanks,
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