How do I use an equalizer?

Answer 1

I assume you mean an audio equalizer for a sound system. To properly use an equalizer, you need to have a frequency-selective sound analyzer. Some equalizers have these built-in. A microphone picks up the sound at the listening position and you adjust the equalizer to get a uniform response over the audio range. Some equalizers will do this automatically.

Answer 2

Other guys got it but ill put it in simpler terms you have the bass - low pitch, the Mid-middle pitch and the trebble- higher pitched when you turn the bass up it makes a more basslike sounding effect to the song the trbble a higher more peircing sound and the mid kinda works both ways down is bass up is trebble.

then there can be middle bass middle trebble etc.

also you might see somthing like this if your doing EQ on a ZEN product:
80 (Bass)
250 (Mid Bass)
1k (Middle)
4k ( mid trebble)
13k (trbble)

same rules apply just in number form