Tips for using a microphone
1. take notice of where the speakers are placed around the room before you begin to speak. That crazy squeal you've heard before is called feedback, and is caused by the soundwaves from the speaker travelling right back into the mic. Avoid tilting the microphone at an angle that creates feedback.
2. keep the microphone directly in front of your chin, pointed towards your mouth. Every time you double the distance from your mouth to the sensor in the mic, the ability of the system to do it's job is reduced by 50%. This creates problems for the sound tech. Most people are intimidated by the sound of their voice over a sound system, and often pull away from the mic. Avoid this temptation.
3. where possible, make eye contact with the sound operator shortly after you have begun speaking. If they're having problems picking up your voice, they will likely be motioning frantically for you to "eat the mic".