although you have a basic question, the answer is really complicated with a lot of study involved. However, here are some things that can help you start with that.
the most important tip I can give you is the thing most people do wrong, and that is boosting a lot of frequencies through EQing to make things 'better'. This is actually the wrong approach, and most people will like it because most people will perceive a louder signal as a better sounding signal, and with boosting you make parts of your sound pallet louder.
The bad part of this is that this will destroy your headroom in your sound and actually will make it a lot worse. Think like this, if you are a painter with a blank canvas you can paint your picture with green paint. if you keep filling the canvas with green paint, eventually their will be no room left to paint anything else in one of the free blank spaces. It's exactly the same with EQ'ing and headroom. If you have an instrument, then that instrument has it's own natural range in a sound pallet. This is referered to in Frequencies (herz). When you dive in to the theory of this you will discover that you have fundamentals in the sound that refer to specific frequencies, like for instance 440Hz is an A note.
Now the only thing you have to do in the beginning is 'cutting' the frequencies you don't use in an instrument. This way you make room (blank canvas) for other sounds to shine through. This way you can paint your complete sound pallet and once you understand how to do this, it's one of the most fun things to do whenever you have recorded your song (or by setting up your style with the correct EQ settings). Only use the boost frequency if you want to have a focus on that specific frequency.
here is a standard table with the frequencies per type of sound
Sub Bass (20-60Hz):
These super low frequencies are the lowest frequencies in the range of human hearing. You might hear these present in sub-bass or super low-pitched bass drums. Sub-bass frequencies are very powerful and require tons of care. It's very difficult for our ears to pick out sub-bass frequencies on speakers that don't have subwoofers, so make sure if you choose to EQ them on your system that you can actually hear them.
Low-Mids (60-250Hz):
The low-mids provide warmth and fullness in a mix. Here you'll find the bass and kick, as well as the lower frequencies in guitars, vocals, synths, and keyboards. This range is necessary to make mixes sound big, though it requires care to get rid of the mud.
Mids (250Hz-1.5kHz):
This specific frequency range is at the center of human hearing. Boosting an instrument in this range of frequencies can provide presence. However, too much boosting in the mids can feel overwhelming to the listener.
Upper Mids (1.5-6.6kHz):
You can give your instruments presence and clarity in the upper range of frequencies. However, as with the mids, you must be very cautious when heavily boosting the upper mids. Too much of this range of frequencies can make your track difficult to listen to.
Highs (6.6-20kHz):
The high frequencies are where you'll find brilliance and air. Boosting these treble frequencies can help acoustic guitar shimmer or vocals sit atop the mix. However, you'll also find tons of high-frequency noise in this range. Electric guitars often have lots of hisses up here, which can sound harsh when boosted.
Practice a lot in the beginning with these basic tips to get understanding what kind of effect the EQ system has on your total sound.
Then you can dive in to the theory behind it and do the more advance stuff. In that case look for guides on:
Parametric EQ
here you will learn the basics behind Frequencies, Gain and Q factor (that's the spread of you gain setting, so how much of the surrounding frequencies are also effected by that gain setting)
Filters
this is also an important part because this can drastically change the sound of your instruments
Masking
with masking is meant that instruments have an effect on each other (phase cancellation), so a common mistake is to use a deep kick sound and a deep bass sound, in that case the low end will sound weak and you need to put a subtractive EQ on the frequencies that overlap in either the kick or the bass (depending on the effect you are after)
A good source to start is the Izotope website, there is a HUGE amount of free tutorials and guides over there ranging from beginners till experienced mastering engineers.
two good points to start there are:
https://pae.izotope.com/and
https://www.izotope.com/en/learn/audio-mastering/equalization.htmlhope this helps you to get started, there is so much to learn about this area, above tips are just the starting point of this very interesting area, but it's really a good skill to have and to practice it a lot, because it's so much fun to hear your source material shine through.