Threshold - the level at which the compressor kicks in
Ratio - the amount of compression applied to a signal that goes over the threshold
Attack - how quickly the compressor handles any transients over the threshold
Level - the adjustment of the signal overall after it leaves the compressor.
(These are usually the settings, but even these can be different, as there are upward and downward compressors)

The compressor works in this way-
For example the threshold is set at -12dB
This means any signal going over -12dB wiil have compression applied, but how much?
The ratio decides how much; at 2:1 if a signal goes over -12 by 6dB (for example) at a 2:1 setting the compressor will make the 6dB over become 3dB over, so instead of going over the threshold by a certain amount the compressor will cut that signal that goes over the threshold by the ratio chosen. Everything under the threshold is untouched.
The attack decides on how fast the compressor tries to adjust those signals. Often very quick will work, if it starts sounding funny, adjust.

There is no magic setting; it will be determined by the mic, the preamp, and mostly the performer. She may need a threshold of -6dB with a high ratio, and quick attack. This will act more like a limiter, which is really what you want.

Make your sound your own!
.. I do not work here, but the benefits are still awesome