Hi,

Make sure that the "input gain" knob is turned up, it's next to the input jack.

If you have a mic that requires "Phantom Power", you'll need to turn on the little button above the input jack that says "48V".

You may want to try using the XLR out to your amp instead of the "guitar thru". You'll probably need an XLR to 1/4" plug for this.

Some amps have a "power amp input", this may help too.