I think the first thing to do is start to be able to identify what is competing for space, to then decide how to handle it. And by space, not just in terms of where something sits in the mix, but whether it's competing for *attention*, and from there, then you can get a feel for whether it's a mix issue, or an arrangement issue (you can fix some arrangement issues with the mix, of course, eg, dropping parts out or changing them up with processing or positioning etc)
Think in terms of left-right (panning), up-down (frequency & volume), and back-front (frequency, volume and ambience).
Should something be up front and in your face, or pushed to the background to give space for other things up front?.. etc. If you've got multiple things going on, where should the focus be? At any time, is one thing more important than others? If so, are you shining a light on what's important, and letting the other stuff give way? A good mix always keeps the important things in focus, but then often brings other details in and out for little moments and colour when the vocal is out of the way, for instance (using automation).
You can do a *lot* with panning, EQ, volume and reverb wet/dry balance...
You can also do some more advanced techniques like sidechaining parts so that, for instance, when the vocal comes in, it pulls some of the instruments that would otherwise compete in that space down a little, or on reverb so that when the vocal is present the reverb levels are low, but when the vocal stops the reverb swells back up a bit - there are lots of tools and techniques to learn, but it's important imo to start with the fundamentals and learning to build a good mix without a bunch of other stuff going on that you may not be totally in control of...