Depends what you want to compress. Good starting point for anything is 2:1 to 4:1 ratio with threshold set so that you get 1-6dB gain reduction.
It's a compressor - play with it ... also depends what you want it for, I use vocal through 1 channel and guitar through other.
Vocal
Trigger -18
Gate off / Release off
Threshold -8
Ratio 4:1 - Low contour off
Auto attack & release
Interactive knee on
Voice - use de-esser
Peak lim +5
Guitar
Trigger -10
Gate on / Release on
Threshold +8
Ration 5:1 - up to 6:1
Low Contour off
Auto attack & release
Interactive knee on
No de-esser
Peak Lim +9
Worth playing with trigger (gate/release (on)) for guitar. I find play with this until you get the cut off you want - this for me is a personal thing.
enjoy and play ...
use auto button for attack and release times, about 4:1 ratio, set threshold so that you get some gain reduction.