When playing live, hopefully the venue will actually provide a PA system.
When they do, you just send everything(drums, guitars, bass, vocals whatever,...) through the mixer, and depending on the specs of the system, every musician might actually get a differently mixed feed from the monitors.
As a guitarist, you can use your own cabinet as your monitor. In stead of putting it behind you(which looks cooler of course), just put it next to you, or tilt it back and put it in front of you. Not only does this give you a pretty good feed of your own sound, it'll also keep your sound from spilling into the crowd too much, which can result in nasty sounding phase problems with the PA.
If the venue does not provide a PA system, you will probably be able to get by by not amplifying the drums, guitars and bass(if your guitar amps have the power), but obviously your singer will need some help.
But don't all singers have a vocal PA to begin with? How else do you rehearse?