Set all "level" knobs to zero, including "gain," channel level and master volume control.
Set all EQ and pan knobs to 12 o'clock.
Plug in your audio sources (microphones, instruments) to the channel inputs.
Play the instrument or sing into the microphone at the loudest volume you'll be using. While this is going on, slowly bring up the gain control. You won't hear anything coming through the speakers or headphones, and that's okay. At some point, a little red light near the gain control will come on. It indicates "clipping," which is the level at which your incoming audio will produce distortion. Back off the gain control just until the light goes out.
Bring up your level controls on each channel about mid-way. Then ease up the master volume control. Watch the master meter and avoid clipping.
Add effects and EQ to suit your tastes and the sound you want to achieve. A general rule of thumb is that more effects make a source sit further back in the mix, less brings a source forward. For instance, go light on lead-vocal effects, so the singer won't get lost in the mix. Try to EQ each source differently, so the individual instrument or voice will be heard.
Open up your mix by using the pan control on each channel. An example would be lead vocal, bass and drums in the center, rhythm guitar hard right, lead guitar hard left, backing vocals spread left to right.