Just for clarification - you plug the mic into a channel on the mixer (part of the PA), which has an on-board preamp. Of course you CAN use an own preamp as well, but it's mostly not necessary.
When you plug in the mic, you set the gain knob on the channel to a level so that the pre-fade level (PFL) is hitting around 0dB on the led meter (or whatever level indicator there is) when singing/playing into the mic fairly loudly. (Push the solo/PFL button on the mixer to see the PFL volume of the specific channel). The volume you set on the gain knob is the volume the pre-amp in the mixer sends out to the volume faders and other stuff. Once you have set the gain knobs right on all channels (i.e. the PFL levels), you use the channel faders to control the actual volumes. Be careful with setting the PFL levels on mic channels, as a too high PFL level makes the mics more sensitive to feedback.
Don't know if you needed this info now but I thought it would be good to know anyway.
