You're using these devices to make you sound louder without spending a lot of dosh, not to put yourselves out of business. If you're putting everything through the PA, then your type of music will dictate the ratio - country will require very little, heavy rock a lot. The threshold control sets a point at which compression comes in and leaves everything below it uncompressed. Attack and release controls let you control how the compressor deals with a signal, how fast it reacts to signal increase and how fast it returns to normal. Be careful with these controls, you can lose the ends of notes and all sorts of nasties like that - so, once again, take your time.
Basic controls are threshold delay and rate.
Threshold sets the level of signal at which the gate opens and shuts. Set it too low and you lose the ends of notes, too high and a decaying note will go on forever. Compromise is the name of the game. A long delay and slow rate on vocals and a short delay and fast rate on drums is the general rule of thumb. There's not a whole heap you can do with a basic spring-line reverb other than control its rate, and occasionally its tone, to get a not too obvious overall 'spaciousness' to your sound.