Help please from you musicians

[ranger121]

Much as I hate to admit having a problem (being a control freak):

We are a relatively inexperienced (technically) 5 piece band, 3 guitar, drummer, vocalist.

The singer has bought a monster sized mixer amp and the dogs in speakers, but none of us know how to get the best sound out of it.

The 3 guitar amps are all mic'd separately into the mix, 3 vocal mics, and the drummer has 3 mics through a notepad mixer into one channel.

What we all want to know is, what is the best procedure for setting everything up so that we get a good sound? Do we play instruments one at a time, get that level right, then two together or what? How is this mixing lark done? What about fold back? How do I set that up so feedback isn't a problem?

(Oh, I forgot to mention, all the wives/girlfiends don't have the first clue either, so it'll have to be one of the band who sets it up.)

TIA for any help.