If every frequency was supported equally in a room, feedback would sound very broadband like hiss or white noise. However the physical room itself vibrates, called a room resonance, which makes that frequency sound louder. When you turn up the volume and get a squeal, that's where it'll be!

Doing live sound is also called sound reinforcement, because that's what you are doing, reinforcing the sound that the room isn't, so that between the two all frequencies are heard equally (a flat frequency response).

The key to getting the most volume without feedback (called 'gain before feedback') is to use equalization so you are amplifying less of what the room does by vibrating. You do that by "ringing the room".

A graphic equalizer is the most helpful, with lots of frequency bands to work with. I start with the mic that is the weakest link in my opinion, and that's the wireless lav mic. If I don't have any of those I use the podium mic. Turning up the sound system GENTLY until it starts feeding back. (It is best that no one other than AV crew be in the room as this can be disturbing to a client who doesn't understand!) Then start adjusting controls until you can make it go away, which is only something you can learn by doing. The more you do sound, the faster you can nail it. Once you stop that ringing, turn it up more GENTLY until the next one, and so on.

A spectrum analyzer can show you exactly where the ring is quickly, and thanks to the ipad there are apps that turn it into one. I have one I paid $15 for, and it provides 1/6 octave increments, and can generate white or pink noise, and all sorts of tones. The only weak point is that if you use the mic in the ipad, the low frequency response is lacking in the mic, which will throw it off, but just trust your ears. Another way to get a general idea is to plug the room dimensions into a "room mode calculator", of which there are several online. It's not something you can calculate out easily manually because frequently the main resonance frequency isn't the problem, it's when harmonics relative to each axis (height, width, length) clump together at the same frequency that there's a big spike.

A pleasant side benefit is that with the room response flattened, everything will sound much more natural as well.