How can I avoid stage fright for a musical performance?

I have played guitar in front of crowds before, and I get nervous but it’s not too bad. The problem is, this time, I want to step it up and sing while I play guitar. I’ve never sung in front of people before and I am planning to perform at a coffee shop next month. How should I prepare for this, as I don’t really have time to gradually get used to it? I am thinking of backing out, as I know my voice will sound awful if I am not confident.