It's not a science, but an art. It's all about what sounds good to you.
Generally speaking, the sliders on the left side of centre are fow the lower frequencies (ie bass), the ones in the middle are for mid-range (human voice is around 1k), and the ones on the right are for the highs. Usually the most annoying frequencies are near the mid range, thus most peoples home stereos and such have a v shape to the eq.
Too much bass will make it sound muddy, too much treble and it will sound tinny. Like I said, it's an art. I have been mixing sound for about 7 years now, and am still learning!