Why am I getting feedback out of the wireless microphone?

The first thing you should do if you are getting microphone feedback is to go to the back of the mixing board. You will see a knob that states 'Mic Trim'. Turn that down, usually to around 9-10:00. It depends on the board, but usually if the trim is up to 12:00-1:00 that will be too high and will lead to your microphone having feedback issues.

The microphone trim works as a master volume for the microphone, so when you turn that down, you will turn down the microphone's overall volume. This may mean you'll need to turn up the volume knobs on the board, turn up the master volume or turn up the volume on the speakers themselves. Turning these up will not cause feedback in the same manner the microphone trim will, however they can cause feedback if they're really cranked up.

If the microphone is directly in front of the speakers, it will cause feedback too. Either use your body to shield the microphone from the speaker or (preferably) don't speak or have anybody speak into the microphone if they're front of the speaker.