Can I throw in a particular bugbear? Whatever system you get, it's likely to have a reverb in it somewhere. Use it like a powerful medicine: remember it's actually a poison! So small doses, and not at all unless you absolutely have to. Too often, reverb controls are used as if they were an echo generation effect, which they aren't. The reverb is there to take the flatness out of amplified sound where the characteristics of the room (carpets, upholstery etc.) deaden any natural reverb. Too much reverb will always muddy the sound and cause feedback. Too little is far better than too much. Turn it up a little as the room fills up, because human beings have the same effect on sound as the upholstery.

Monitors: I'd suggest doing without these at first, especially if you're likely to be playing as well as setting up the sound. From behind the speakers (where you should be when you're playing), you can't get a true impression of the PA sound out front, but at least you have some impression. With practice, you'll be able to judge from what you hear when you're playing, roughly how it sounds out front. If you've got monitors, you'll hear them and nothing else, but the sound from the monitors can be nothing like what's coming out of the PA, particularly with small-scale gear where you may have little or no EQ (tone controls) on the monitors. If you're happily listening to yourself through the monitor thinking how great you sound, and the out front starts to feed back, the first you'll know about it is when the audience run for the door with their hands over their ears.

To qualify all that, if you've got someone else looking after the sound while you're playing, use the monitors.

One other thing I'd suggest is, always try to give yourself at least a short sound check. You're better off starting 15 minutes late, and giving yourself half a chance to sweeten up the sound, than having to stop half way through your first song to sort out the caterwauling emanating from your speaker cabs.

And good luck; I thoroughly enjoyed learning bit by bit how to set up PA systems, and it's very satisfying when you get everything just right – which you will!