knowing what each setting does, and hearing what each setting does is different. from a quick google search and a 10 second read...this is what i got...
"work from the extremes backwards
ie: start with attack and release at the shortest settings, the ratio at the highest and the threshold low (you might need to adjust the makeup gain at this stage)
it'll sound very ugly at first, but as you increase the attack and the release, you will hear exactly what those parameters are doing
then decrease the ratio until the sound is the right size
raise the threshold until it sounds right (reduce the makeup gain)"
if you do this on the tracks you want to compress, you'll find out what the different knees, attacks, releases, and ratios really do and sound like for different instruments. it will eventually let you know what the track needs. it just takes a consistent method and a little practice.