Based on what you are saying you probably are a perfectionist when it comes to many things, DJing being one of them. It’s good that they have an expectation that you won’t be perfect. Just remember that we all had to start somewhere. You don’t have to be perfect with beatmixing or song selection.
It’s always smart to ask clients for songs that THEY would like to hear that way you don’t have to make a bunch of assumptions right off the bat.
The one part I would recommend knowing how to do is microphone and speaker set up. That part you need to know how to do because if that doesn’t work then that’s a bigger issue.