I was totally lost with compressors, until an engineer friend pointed me in the right direction.
The simplest way to use a compressor, is by adjusting the basic controls in a certain order.
The basic controlsbeing ATTACK, RELEASE, RATIO, THRESHOLD. This is the best order to adjust them in.
Firstly, set the ratio as high as it will go, set the release as fast as it will go, run your audio through the compressor, and listen carefully to the attack. It will probably sound terrible, but concentrate on the attack only, adjust it until you are happy with the attack
Secondly, Adjust the release, generally the slower you can get the release, without it booming or pumping too much.
Thirdly, the Ratio, generally the higher the ratio the smaller the range of the sound, this is the part that "squeezes" the sound, play with this until you are happy with it.
Fourthly, the Threshold, this basically allows you to choose the amount of uncompressed sound that is let through, you don't want it to be compressing all the time, fiddle with this until you are happy with the balance of compressed and uncompressed sound.
This is only a rough guide that I use, and it works for me, but if you are in doubt use your ears, sometimes it's hard to tell what a compressor is doing, it's just a matter of training your ears.
Damn I hate typing.