In the best of all possible worlds, the club's sound guy would do nothing more than adjust the stage mics to provide an appropriate volume and use his/her standardized EQ settings to compensate for the "room oddities".
Too bad that so many bands don't really know how to "balance" themselves and use reasonable stage volumes.
Since you guys haven't played out much, and since I'm assuming you don't have a "club sized" practice space, do this...
Arrange to borrow or rent a space at least as large as a very small club and a small, basic PA system. Often schools and churches have "activity" rooms you can rent for an afternoon at little or no cost, but even something like part of a warehouse will do. Don't worry too much about the acoustics of the space.
Use the PA system only to provide vocal mics.
Have the drummer play at his/her normal volume. Then you and the guitarist need to adjust your amps so you blend properly with the drummer. Not too soft...not too loud. And remember, back in the day, this was done with 40-watt Fender Bassman and Super Reverb amps. If you or your guitarist need to drive your amps hard to get a certain tone that's okay, but only use the volume that is absolutely necessary to get that tone. Start with the volume down low, then turn up only as necessary.
Then, you and the guitarist need to get down on your knees so your ears are at speaker level, and adjust your tone to exactly how you want it. Don't worry about how your tone sounds in the room, you're only interested in how it sounds coming out of the amp speakers!
Run through a few songs and make sure you have both your volumes and tone settings they way you want them. Again, ignore the "room" sound; only listen to the sound coming out of your amp's speakers. Don't worry too much about the vocal levels; the vocal mikes are there just for "comfort".
Then, you and your guitarist should write down every single setting on your amps! These will be the settings you will use at your gig.
Then consider exactly how many and how large the amps you will need for the gig really need to be. A wall of Marshall cabs can be really impressive looking, but do you really need all of them? It's okay to have enough gear on the stage to provide the desired visual, but they don't all have to be plugged in! A minimalist approach will speed up your setup and tear down, effectively give you a larger stage to move around on, and save your back a lot of grief.
On gig night, set the controls on your amps to exactly what you wrote down during the "dry run", and leave them the fuck alone after that! Do not tweak or touch anything! Don't worry about what it sounds like on stage!
You'll be giving the sound guy a sound that is easier for him to work with. And since he's not having to fix a crappy sound setup by the band, deal with people turning their amps up and down, and crap such as that, he'll be able to focus on giving your band a good sound in the club.
Give the sound guy something good to work with, make his job easier, and you'll end up sounding good. And in addition to introducing yourself to him before the gig, make sure you thank him and maybe buy him a beer after you finish your set...
