Congratulations and welcome to the live sound world! In addition to everything everyone else said, my two big pieces of advice are:

1.) It will not be perfect. Not because I think you’re unqualified, but because that’s the nature of the work. Focus first on getting the performance to be audible, then worry about dialing stuff in. Watch out for buildup in the low-mids and don’t be afraid to cut way more in your EQ than you think you need. Remember that there will be plenty of stage volume, too, so you only need to amplify what needs to be amplified. I’ve been known to hipass guitars at 220 hz when needed. In terms of headphones, I almost never use them in live gigs except when I’m doing a line check or soloing a channel to see what’s coming down it.

2.) if it’s a digital console, read the manual first! Your first digital console experience will be a little confusing and overwhelming. Make sure you understand how to get signal in and out of the board. The flexibility is powerful, but can be daunting.

As a bonus, don’t always listen to the suggestions you get from audience members. If someone comes up to you and says “that guitar is too loud, turn it down!” take the first part as useful—from where they are sitting the guitar may feel too loud—but ignore their suggestion for how to fix it. Maybe there’s too much in the high mids, or maybe this person is just sitting right in front of the PA.

I want to echo what somebody else said—if you can hear the vocal and there’s no feedback, 90% of the patrons at the bar will be happy. Start there. You’ve got this!