If I were to suggest one single piece of advice it would be to mix at lower levels - we all get off on our own music and the temptation to crank-up the levels to stadium band level is compelling, but the louder it gets then the more subtle elements of the music get drowned out.
And now I'm on a roll I would also suggest getting the basics down first and scrutinize them, eg drums, bass, guitar/piano/keys - those 3 represent the essential components - rhythm/melody/harmony, go through them phrase by phrase, bar by bar and make sure they sit well together. It is a tedious exercise but you might well come across notes that are say a half beat or so out, you could get away with that with say a string part, but not with the basic elements, it'll sound slushy, additionally listening carefully to those elements you can experiment with different arrangements - for example if you have a F1 note in the bass, why repeat that note in say the keyboard part, consider inverting it, or adding say a 7th etc. This widens your musical vocabulary, just in the same was as just saying something is 'sh*t' you'll have the notes to express yourself more effectively.
Once done look at the engineering side EQs/Compression etc - and well that's one of the dark arts, I am still lumbered with the logic of More's Law - if more is better then too much is just right, I still tend to think if wazzing some control up to say position 1, and there is some enhancement, then by default I crank the control up to full, then come back the following day and wonder why what previously sounded encouraging now sounds like a 50 oil gallon drum full of ball bearings falling off the edge of a cliff, bouncing it's way to the bottom, accompanied by the sound of a cat scraping its way down a blackboard and the lot coming to a horrendous clash at the bottom when I play my final chord.
If only I could listen to my own advice when I say there is something to that "less is more" wisdom
Listen a lot to the sort of music you hope to be inspired by. I did one piece of music that I tried to improve by adding more parts (instruments) then one day I was noodling about and decided to reduce the tune to just finger picked guitar, bass and 2 voices, one lead, one harmony and that improved the tune no end. Stripping the song back focused my attention on the vocals, the alliteration, the phrasing, the dynamics etc the guitar part, not being smothered by keys, strings, pads etc came out much more convincing and because the bass also stood exposed, it provided for a more lyrical contribution instead of just plodding along playing root, third and fifth. I was playing the instruments instead of just adding notes, the musical equivalent of painting by numbers I guess.
I guess from your post you are writing your own stuff, so are you asking how to improve your compositions or are you asking how to produce the tunes, or asking how to engineer the result? It's a big ask, being an expert in all, but some people do come up with very convincing results, so it's not beyond the bounds of all possibilities.
One of the problems of writing on the computer is that we produce the 'breeze block' equivalent of music - how many of us have hard drives festooned with 'kule' 2/4/8 bar loops, but none of them ever finished. Resist the temptation to compose at the computer, unless you are doing say EDM where the phrasing and structure is more predictable.
I suppose the whole thing is like baking a cake, the icing etc only ever goes on last, but if you got a 'soggy bottom' all the icing in the world won't save it.