How strong is the mic-signal coming in the mixer? If available on the mixer, press the PFL-button for the channelstrip you plugged the microphone in (make sure you disable PFL for all other channels). Watch the LED-bars. How many leds are lighting up when you sing into the mic? The more LEDs lighting up, the stronger the inputlevel. Turn up the gain knob for the microphone-channel to get more LEDs lighting, making sure you never let the red LEDs light up!