Feedback is the tortured howling that results when the output of your speakers gets fed back into your microphone, and is amplified and sent back to your speakers in an endless loop.
Feedback is the Darth Vader of amateur sound reinforcement. To prevent it, take these steps:
Use a unidirectional microphone, which rejects sound coming from the rear of the mike.
Direct your microphone so that it doesn't pick up speaker output. Because they can placed more purposefully, hand-held and stand-held mikes give less feedback than clip-ons (which I still prefer for the freedom they give).
Place your speaker(s) as far forward of your performing position as possible.
Use an equalizer to reduce amplification of the particular frequencies that feedback in a particular room.
Musicians tend to need several microphone inputs, so good-quality portable amplifiers usually have a mixer built in, to accommodate the signals from several microphones. These Mixing Amps come in heavy-duty cases, cost $300-500, and do their job quite well.
The crucial feature of your mixing amp is a built-in graphic equalizer. For the storyteller, this has one purpose: to reduce feedback.
An equalizer is like the tone control on your car radio, which lets you boost or reduce the high frequencies. A graphic equalizer, however, has a row of vertical sliders, each of which controls the level of a particular frequency range. The more sliders, the more precise control you have over which frequencies you boost or reduce. The minimum you need is five; seven or nine are even better.
When you set up your sound system, speak into your mike while slowly increasing the volume until you hear the first hint of feedback. Listen to the pitch that is feeding back. The room you're in is echoing most loudly on that frequency. Experiment with the sliders on your equalizer to reduce that frequency. Now you can have more volume, without feedback or boominess, and without reducing all the high frequencies in your voice!
If you're having a problem with hum, or with popped "p's" and "b's," you can sometimes use your equalizer to eliminate it by reducing some of the low frequencies.
Choose your portable mixing amp by the graphic equalizer. If the equalizer is what you need, the power of the amp and the number of mike inputs will almost certainly be adequate.
In a previous article on microphones, I recommended a wireless clip-on mike for most storytellers. If you tell sitting down, however, you might not mind the trailing cord of a wired clip-on. Of course, some tellers prefer hand-held mikes or mikes on stands. These cost the same as their clip-on cousins: about $125-150 for a wired version, and about $500 for a true-diversity, wireless version.
In all cases, you should probably get a uni-directional element, to reduce feedback. Mikes in this price range are also low-impedance, which guarantees that you can use long cables without adding hum.
Most musicians use two speakers for sound reinforcement, one placed to each side of the performer. For a single storyteller, however, you only need one speaker! Put it to one side of you, angled to be heard by the whole audience.
If you can only invest $350 in speakers (the minimum), it's much better to get one good speaker than two poor ones. A good speaker is about the size of two stacked apple-crates, and weighs about 50 pounds. Compact speakers exist, but they have reduced sound quality and/or power-handling capacity, so for size and weight you are again better off with one good speaker.
When you set up your system in a new place, experiment with speaker placement. You want it in front of you, not behind you. Ideally, you want it off the floor (you can buy a special stand to hold it at eye level, but I usually improvise with a school desk or other small table; or else I leave it on the floor). Typically, you'll angle it toward the center of your audience, but in a gym or other boomy room you may do better to bounce the sound off a side wall. Try speaking into it as you turn it from side to side; choose the direction that sounds best.