Try panning all your instruments first, sometimes you won't need to e.q. But E.Q. can be corrective or creative. Corrective basically cuts any unneeded/annoying frequencies out. Or it cuts out any frequencies that are clashing with each other to improve the harmonic balance of the track. Creative E.Q. adds color and can completely change a sound and add lots of character.
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To start..
cut everything that is not needed.
Turn your Q as high as possible. then turn the gain to 10-18 dBs. Then sweep across looking for any annoying frequencies. then turn the gain to negative and see if it sounds better with the cut or not, play with the Q a little. Cuts are usually narrow.
After a while, you will be able to listen to the track and estimate the frequency that you want to cut, and then sweep around that are. You should sweep for everything and bypass things to see if you are really improving the sound or just making it louder (easily confused amongst beginners)
Vocals, lcf/hpf around 80hz,
Piano, around 100hz or so
bass, maybe around 30hz