First, welcome to the Rabbit Hole. Plugins are wonderful, no cords, no weight, no physical space requirements. They can be overwhelming, so many choices.
There are umplty bajillion great EQ plugins. Somme work differently than others but in the end cutting and boosting frequencies is pretty cut and dried. It is a paramount factor in getting a good mix.
It is important to remember that sometimes you won't need much/any EQ and other times cranking/cutting can make a big difference in a positive way. If possible, less is more and subtracting is usually more subtle than boosting. None of that matters - use your EARS!!!
I recommend you just play around with one track first. One plugin I use often is included in the Apple system software - AUGraphicEQ. I recommend it because it is very simple to use and very specific regarding effecting a change in frequency response. It will provide an excellent overview and make it much easier to understand any other EQ plugins that you may encounter.
Drop that into a track and then play it back while moving the sliders one a at a time slowly all the way up and all the way down. You will notice that some sliders don't seem to do anything, that usually means your track does not contain and audio information at those frequencies. The other possiblity is your playback system cannot reproduce those frequencies. This usually happens with low frequencies and you probably won't hear much below 40hz or so depending on your system - which does not mean there is nothing there.
After listening to each frequency, flatten out the EQ and listen to the track. Try playing with various combinations of the sliders, most good EQ ends up being a general set of frequencies rather than spiking or dipping a single specific one. The GraphicEQ plug in allows you to use the shift button and select a range of frequncies, they will all move up and down with each other. Play with that and see what it does.
If you think of something, try it and listen. This is all about learning how that particular instrument sounds in the various frequency ranges.
You may see your DAW output meters shooting way up past zero when you boost frequencies. If you push a low frequency band up and the meter pins, that means there is information there. If you can't hear it, that is your playback system, the sound is below it's ability to reproduce. It may still try so be careful not to blow your woofers or headphones. I often put a plug in behind the EQ that has an Input gain control so I can bring it back down in the channel. My personal choice for that has usually been Tracktion Master Mix (which also has great pre and post EQ and a compressor). There are lots of options, some are probably already on your computer, use one of those
Once you have an idea of how the EQ works on an individual track, un-mute the rest of the tracks and start playing with using EQ in a full mix. This is where you want to live with EQ, as a tool to improve a mix.
The individual track exercise is just to gain familarity, mix with everything playing for the most part. As they say, this is where the rubber meets the road.
As a general rule, there are usually at least 2 instruments that need to be sorted in the low frequency range. You really want only one instrument to carry the deep low end, more than one just makes a messy, mushy mix.
It is your choice which instrument you want to hold down the low low end. Often the choice will be between Kick and Bass instrument but that is not written in stone.
Was learning how to use white noise and an RTA to dial in a club for live sound once and the soundman then played a Prince record. The snare had the most low low frequency response on a couple of tunes and it sounded great. The kick and bass were in different non-competing bands up a bit higher.
If there are vocals, those will be the primary focus. They shouldn't need much low frequency, carve it away so it doesn't clutter up the lows and find where it sounds the best, all voices are different and all can sound different depending on the mic used, the distance from the mic and the style of singing. None of that matters, use your EARS and find a good EQ for the vocal.
With low frequencies sorted and the vocal in place, the rest of the instruments can be EQ'ed to sit in the mix without getting in the way of anything. If you can get a great sounding mono mix, you are golden. Then you can pan things out of the center a bit and everything should be sounding great.
It is a process, it won't happen quickly. Be patient, learn and experiment. Most important, use your EARS!!!! Cheers, Kuru