There are three main aspects of EQing that spring to mind.
One is EQing a
system to a room.
Second is EQing monitors for foldback
EQing individual elements (channels) to make them sound right.
It's a hugely
complicated topic that can't be covered in a post.
Have a read of Hugh's very
informative article Here
But here's a few thoughts to get you going.
Room EQ: if you have access to a system and a 32 band graphic EQ, play some of your
favorite tracks through the system and then bring up each band on the EQ to see how it
effects the sound. This is now my preferred way of EQing a system. I have a handful of
tracks I know inside out that cover a full frequency range. If they send good through the
system, the system will sound good full stop!
Monitor EQ: is often a compromise
and a means of (often almost deaf) performers hearing themselves on stage. Feedback can be
an issue so it's a good idea to be able to recognize frequencies as they feedback.
Channel EQ: the first step in adjusting the EQ of a channel is to know what the
instrument should/could sound like. So often I've heard sound engineers degrade an
instrument's sound simply because they really don't know how it should sound. Ever heard a
string quartet where every instrument sounds like a violin? Once you know how it should
sound, then you need to learn what is required to adjust the EQ to get there. If you don't
know how it should sound then it's a little like having a map with no destination in
Random but hopefully useful thoughts.......