Set up your PA system. Take your two main speakers and face them out, away from the band. Attach one speaker cable to each main speaker and run that cable back to the mixer and into two of the line-out inputs. Connect a microphone to the mixer and place into channel one. Turn on your mixer.
Test the vocals through the two main PA speakers. Have one person stand out in front of the PA system where the crowd would stand, and have someone sing through the mic. Adjust the levels on channel one to make sure the vocals are loud enough out in the crowd. Have the full band play while you test the vocals to make sure that the vocals will sound loud enough when the band is playing.
Set up your monitor speakers. Most bands usually use two monitors. Place one on each side of the stage and run one speaker cable from each monitor speaker to the mixer and place them in two of the line-out inputs. Sing through the mic to hear vocals through the main speakers and the monitor speakers.
Test for feedback with the monitor speakers and the mic. Do a sound check with the band again to make sure you won't receive any feedback from the monitors. Make sure the sound is still loud enough out in the crowd, but also make sure you can hear the vocals loud enough in the monitor speakers. If the vocals are up too loud, you will begin to hear feedback. This is an easy fix and only requires you to turn down the output that is coming into the monitor speakers.