Experiment! You can use this cheat sheet (not mine) to get an idea of what boosting or cutting certain frequencies can do to instruments, voices, and sounds. It really just takes a lot of ‘doing’ to get a feel for it. Use your own tracks or sounds, or download some stems from a band you like and have a go at EQing their instruments or vocal tracks (Radiohead released a few of their songs broken into stems awhile back; I’m sure there are others). 

If we’re talking about music in particular, it’s important to understand that while you can and should practice on individual “soloed” elements (listening only to the guitar in a mix, for example), EQing should almost always be done while listening to at least some of the other elements. This is because the other elements impart their own sonic characteristics, and something that might sound good EQed on it’s own may not sound as you intended in context.

Edit: Here's another good interactive chart.