Depending on the size of the place and whether you’re just playing on your own equipment or there’s a rented PA system with a sound guy and microphones for the drums etc…
Drums: You might not have to do anything about them, or you could run them through the PA. Drum mikes are expensive, but the sound guy might have some.
Bass: I’m a bass player, and my amp is too small for anything but rehearsing by myself in a soundproof cupboard on the dark side of the moon, unless I breathe loudly. I usually play through the PA. You’ll need a DI box for that, if the PA is rented (or owned by the place you’re playing) the sound guy probably has one.
Guitar: Electric guitars through their amps. You can put a microphone in front of them and run that through the PA, too. Underpowered guitarists are rarely a problem, in my experience - they’ll expand their Mount Marshall until you don’t have room for the instruments in the room and have to tie them up with guitar strings to make them stop (don’t worry, the strings will break before the first set is over anyway). Depends on the kind of music you play, of course.
Accoustic guitars through the PA, or at least that’s what we do.
Vocals: PA. If the place is very small and you’re not going to play Sound of Silence more than Smells Like Teen Spirit, it might be a small PA, and then you might want to borrow a bass amp rather than let the bass compete with the vocals through the PA.
You’ll want to have a microphone.
Monitors: The Beatles, in the infancy of stadium rock, played without hearing themselves. This was no problem, because people were screaming and fainting to loudly to hear them anyway. For the rest of us, hearing what we’re up to is nice. Yes, you can get away with not using monitors and listening to the PA, but only if you stand in front of the PA, otherwise it will sound muffled, and standing in front of the PA brings feedback problems and isn’t good for your hearing, either (the audience has the option of moving around away from the speaker, you can’t leave the stage). Depends on the music, of course. On the other hand, violinists playing softly get hearing problems too.
The last three gigs I played: The first we had monitors, smaller speakers facing the stage, at the front of the stage and next to the drummer (depending on the size of the stage). They come from the PA, and you can adjust how much of everything you want in the monitors.
We were placed like this (ok, this was a bit hard to do in ASCII, so…): PA speakers at the front of the stage or on the floor, on each side. Two monitors (one each, only three of us) at the front of the stage, microphones a bit to the side of them (to avoid feedback), us behind the microphones (well, duh). Drummer behind us, guitar amp next to him, and his monitor on the other side. I played through the PA, so no bass amp. If we’d had another guitar player, his amp would have been on the other side of the drummer, probably, although who stands where depends on who plays together, sort of.
Second gig was a smaller room, we used our own PA. To save room we didn’t use monitors, and placed the PA speakers on the stage behind us - meaning that we could hear what we played, but we had to be really careful to avoid feedback. Like this:
PA, drummer, guitar amp, PA. In front of the first PA; me, then my microphone (so that I was between the PA and the microphone. Don’t move around too much). Same on the other side with the guitar player.
Third: Played a few songs at the guitar player’s sister’s wedding. The PA really low, the drummer playing with splitsticks on all the songs because they didn’t sound as loud, guitarist playing accoustic through the PA, and absolutely no monitors. Like this: Drummer and PA squeezed in between the wall and the door to the kitchen, guitar player and me on chairs in front of him with a microphone each, wondering what on Earth to do when someone asks us to play waltz. Semi-relevant point being, it wasn’t loud enough for us to worry about feedback.
If you have a sound guy running the PA but also partly use your own amps, let him be boss. If he tells you to turn down the amp or something, do it, but of course let him know if it means you can’t hear what you’re playing. He can hear what the audience hears, you don’t. If you’re too unsatisfied with him, get someone else next time.
For the same reason, even if you don’t have a PA system and there’s no sound guy, it’s a good idea to have someone you know, preferably someone who know what he’s doing, in the room to tell you if you need to adjust your sound - because while the bass sounds like sweet bliss on stage, it might be seriously disturbing someone’s intestines in the audience. Also, you won’t be able to convince the guitarist to turn down, but someone who tells him ‘Dude, in the audience it sounds like a tortured Nazgul’ might make him fight the guitarist instinct.
I’ve often brought along my amp anyway, in case there was a PA problem I’d at least have something, if there was a monitor problem I’d have that, and also you can feel what you’re playing to a certain extent, at least the rythm, if you stand right in front of it or sit on it. On the other hand, I’ve usually brought along a spare battery, spare strings, a spare bass…
Don’t try to sing through the bass amp or anything. Forget it. And certainly not the guitar amp, especially if he uses a fuzz box or any other effect…
Not thud, but THUD
This is not as great a luxury as it seems, because the singer will lose an eye to a poorly aimed pair of panties, and if you’re playing in a hotel you’ll be stoned to death with room keys.
Beethoven, the Metallica of his day, became stone deaf. Was exhausted, too, having to drink, smash hotel rooms and sleep with a whole band’s worth of groupies all by himself.
If we’d had another guitar player, he would have fallen off the stage or had to sit on top of the bass drum and shouted ‘PSSSH’ every time the drummer hit him in the back, because he’d take the place of a cymbal.
A bunch of thin sticks taped together, looks like something stolen from the kitchen.
Unless it’s punk.
‘Ok, ok, I’ll turn down, dammit.’ ‘Uh, Joe, that’s up.’ ‘Sorry, force of habit.’