You can also see my other instructables on sound:
How Sound Works
The Concept Of Sound Pressure (SPL)
Before we go into how to fix problems there are a few things that I strongly believe about sound that you should know.
1. You can't program sound.
2. If it can fail, it will. If there is no way possible for it to fail... then it sill might.
3. You should know your equipment better than the back of your hand before attempting to troubleshoot.
With that in mind, let's talk audio!
Step 1: Feedback
You can fix feedback by turning the volume on the mic down (the fader on the sound board), turning the gain down (usually the knob at the top), moving the mic behind the speakers, or turning the main fader(s) down. The preferred way to fix it is to turn the gain down, or the main fader(s). (The main faders affect your gain structure.) If you don't know what mic the feedback is coming from, you should use the main fader(s). To prevent feedback keep your volume and/or gain as low as possible.