you really do have to just play with it to make it work ... but if it's any help, here's what I do for pop stuff ...
Vocals aren't all that complex (say compared to a drum kit or an entire mix) - and for typical pop vocals, you want to tame those peaks that go over the threshold you set. Myself, I don't want to make my vocals too obviously compressed.
If you use a fast attack (say, 20 ms), it will clamp down more quickly than a slow attack (say, 100ms). How much it clamps depends on the ratio you set. For something like bass, if you use a fast attack, you'll lose all of the attack from each note you pluck and the back end of the note will be louder - which isn't usually desirable unless you want a sustained effect (in which case, you would set a low threshold along with a slow attack and release).
For vocals, a fast attack will tame bursts of volume that make the vocalist sit unevenly in the mix. The release will give you more or less of the compressor's sound - for transparent compression on vocals I try to set the release and threshold to be just enough so that the quiet notes are *just* allowed to go through without any compression being applied. Typically, I'll watch the meter to see how long it's taking to get back to 0 before the next syllable comes. If it's not all the way up to 0, then the release might be too long. If it reaches and stays at 0 for several ms, then it might be too short.
Here's the catch - depending on the music, and the threshold/ratio you set, nearly any combination might work.