You've got your amplifiers, you've been practising in a garage or small rehearsal room and things sound pretty tight. Now you've got a chance to play down at the local pub/bar. How difficult could that be? Well, if you feel that all you have to do is set up and play just as you've been practising then there is a 99% chance of disaster. Many bands sound awful at their first gigs because they haven't balanced their on-stage sound.
You'll have grown accustomed to balancing to the drums when practising but you'll need to be louder for your gigs. The answer is not to simply crank everything to 11. If you are struggling to hear yourself don't get into a volume war with the other guitarist and/or bass player, it's the balance that is wrong. Guitars will need 30 to 50watts RMS to match the drums. The bass will need 50 to 100 watts. You'll notice that most decent combos will meet these needs for small to medium sized venues. The vocalist is the one band member who will not be able to hear himself without monitors. Keeping within sensible levels you can balance the volume of guitar/s and bass against drums, and keyboards if you have them. Each band member can move towards their back line kit to better hear what they are playing, and knows where to go to check the other guys too.
To get balanced, first set up the drums, back-line, the monitors and PA, and make sure everything is functioning. Your band should line up with the bassist to the drummer's left as it's the best eye-line for the drummer who needs eyes-on the other half of the band's rhythm section. Lead guitar is to the bass player's left with rhythm on the drummer's right, again in line of sight and carefully aimed drumstick! Vocalist front and centre. Start with your monitors up but the PA right down and send your sound engineer (or a willing volunteer with good ears) into the auditorium. Run through one of your best songs and check that everyone can hear what they need to hear on stage. Check with the sound engineer for how the instrument balance sounded. You'll need to make some compromises if one or two people are unhappy. Try moving them, or adjust the monitor positions, check the angle that speaker cabs are facing in, you may need to move people away from something or you may need to move them closer.
Guitarists, don't forget that you have a tone control as too much bass will create a muddiness, leave the bassier bass to the man with four strings. You can control this at the guitar. Many guitar players simply set the tone knob to 'max' and forget about it and at this setting you will get the widest range of frequencies your guitar will produce but try being more focused. Single coil pick-ups can, for example, be very bright, sometimes more than you might like; to add some depth simply roll off some top-end with the tone control.
The guitar also has a volume control and it's often used simply as an on-off switch, all or nothing, but it's a variable control, not a toggle switch, it's meant to be versatile, so use it like this: With the volume control at max, set the amp to the highest gain/volume settings you intend to use, now turn down the guitar volume control for a cleaner sound for the quieter stuff you play. When you want oomph, get a smooth change by turning your guitar volume control back up. This is a professional technique much used by great players including Jimi Hendrix. While you are checking those maximum amp settings for gain and volume levels, check them rolled back a couple of degrees, too; you'll often find a punchier and more cutting sound waiting for you. It's all too easy to tip over into a more squashed, thrashy, rock-out sound that is much more exciting on-stage than it is for the audience.
This, then, is the sound check, a time for all to give their opinions. It's very rarely possible for everyone to get exactly what they want so be professional and agree workable compromises; it's no good if people start turning up, or down, later. Both the guy that wants to rock out at 11 and the shy self-conscious player who backs off, share one thing: they can both ruin the mix, your band's sound.
This is the time to agree the on-stage sound and once it's set it doesn't get altered unless problems occur later, such as feedback issues or the crowd absorbing more, or less, of the front of house sound than you'd anticipated. This is being professional. The whole band needs to agree not to change individual settings once you've decided on the balance. The sound engineer is the only person who should adjust volumes once the balance is set and needs to remember that the mix he can hear in his headphones will sound different to someone in the middle of a crowded venue. The engineer, though, should never change that agreed on stage sound balance (unless individuals have been cranking things up!). Communication between band members, and between band and engineer, is essential. If anyone has a problem, use the time between songs to say so and sort it!
Finally, the PA. Everyone imagines this is the sound check - all that "one, two, one two" stuff. It's actually the last part. Remember that the PA needs to be set up so you can adjust the sound separately from that going to the monitors. Run through a song, this time setting the initial volume for the vocals, they will sound loud without a crowd to soak them up and the sound will bounce back from the rear walls and floors. Adjust the back-line if necessary. Run through the noisiest songs for vocals and for guitars, checking what happens when effects kick in or heavy distortion is added. Check the quietest bits too. This is a time for gentle tweaking and fine tuning, hopefully you'll have created the right balance in that first stage above! Play through a couple of numbers and make sure that everyone is as happy as possible. Once the crowd is in you'll need to fine tune some more but with the initial balance right, that's the easy part. Good luck!