Equalization is the art of adjusting the volumes of certain frequencies of a track in a way which leads to more pleasing tonal qualities. Most Daw software comes with an EQ on each channel. There are also EQ plugins available; some free, some not.
If your tracks have been recorded in a good sounding room and with proper mic techniques, EQ may be uneccessary. When you do need to use equalization, there's only one way to get it right; trust your ears. We often hear 'How do I add more thump to the kick?'. The answer is often 'EQ'. This can lead to 'I tried that, but now everything is muddy. What frequencies should I cut to get rid of the mud?. The answer is 'It depends on which frequencies are causing the mud'. The point is; each track is different (based on how it was recorded), and no one can tell you what frequencies to adjust without hearing the track. It's like asking 'How do I get to Nashville?'. The answer depends on from where you start. You gotta train, and then learn to trust, your ears.
So, how do I train my ears for EQ work? Here are a few suggestions to get you started and some great reference material for further study:
Roll off the low end - Much of the 'muddiness' we experience when mixing two or more tracks together is caused by instruments completing for the same space; mostly in the lower frequencies. A prime example is guitar and bass. Guitars produce many low frequencies that can impede a speaker's ability to accurately reproduce the bass. The result is a bass that lacks definition and clarity. One way to solve this problem is to eliminate the very low end of the guitar track. Here's how using EQ... Change one of the EQ bands to a High Pass Filter (allows high frequencies to pass through unchanged while cutting lows) and position the band to remove unnecessary low end. For vocals and guitars, you can usually roll-off below 150 or so. For kick, try cutting everything below 60. For snare and toms, try 120. For bass and piano, around 50.
Carve out space - Instruments that lie in similar frequency ranges tend to 'mask' each other. Solve this by cutting a rather narrow notch in one instrument while slightly boosting the same frequency in the other instrument. Narrow cuts generally do little to affect the tonality of an instrument but can do wonders in terms of allowing another instrument to shine through in that frequency. A good example is bass. 400Hz and 800Hz are important in adding clarity to a bass line. Guitars, vocals, piano, and kick can all compete at these frequencies. It would seem that boosting the bass at 400 and 800 might solve the problem. What we find, though, is that we're better off carving out space from the other instruments and leaving the bass alone. Try it and see what your ears tell you. Try carving a bit at 3kHz to disguise an out-of-tune guitar or vocal. Try cutting 3K out of harmonies to allow lead vocals to shine through.
Cut more than boost - Why cut? Because the human ear is accustomed to frequencies being absent in a natural setting (due to natural room dampening). So cutting a frequency sounds more natural than boosting one. Find frequencies that make an instrument sound good and CUT that frequency out of all other instruments.
Boost & Sweep - How do you find which frequencies to cut? Open the EQ. Raise a single band and SLOWLY sweep it left and right listening for muddy and/or nasty sounding frequencies. When you identify one, cut it. Cut & sweep also works, but because of the ear's ability to adjust to missing frequencies, boost & sweep is usually easier.