To prevent feedback (that howling / screeching sound):

• Keep your mouth close to the mic, not a foot away!
• Speak clearly into the mic – and turn the mic volume down if you are too loud when you do so. What you want to avoid is whispering into the mic, then turning the mic volume up to compensate. This can cause feedback.
• Don’t point the front of the mic directly at the speakers. Try to stand behind the speakers when speaking. Switch the mic off when not in use.