Ayy another Garageband user!
So there's a couple things that you could try doing when you work with the audio in a mix after recording everything:
1) Arrange things strictly with volume and panning helped me get a better idea of where things should go that functionally makes sense. Example, melody typically is more prominent that sound effects or other parts that serve a textural function.
2) Garageband has a lot of pre-made settings for things like EQ that I'd suggest looking into and trying. Learning about where each instrument is located in the frequency spectrum helped me understand how to make different instruments resonate a little better. Choosing these settings that are already made in Garageband is a bit of a shortcut, and would probably best to not completely rely on them in the long term, but it might help you while you're still learning the whole mixing process. Plus, just as importantly, it'll help you understand what frequencies can be removed because they don't add any value to a particular instrument.
This is just my two cents of unsolicited advice lol that I've picked up over the years, but I hope this will give you some next steps as to how you can improve what you've done here.