A microphone is excellent for projecting your voice so your speech is clear for everyone in the audience to hear. Unfortunately it will also transmit other noises it picks up and these can be very distracting for your audience.
A few examples:
You should also be careful to keep your notes on the side of the microphone away from you. When you hold your notes between you and the microphone you create both a barrier for eye contact with your audience and a barrier for the passage of air between your mouth and the microphone. It can sound to the audience as though you have your head wrapped in a towel!
In the next post I will talk about moving around when using a microphone.