Composition and melodies are fairly well done actually. The transitions from section to section do feel a bit abrupt at times, using some leading / trailing effects could help with that, as well as giving some melodic indication.
Sounds quite dry, can tell not much processing has been done on the mixer (if garage band even has one anyways)
Not too sure about the sound design. There’s this one violin note (the highest) that keeps bothering me. Don’t know if it’s off-key or if it’s otherwise tonally distorted by the plugin. And the synth choices are quite… interesting. Not too sure what to think of them, as they do seem kind of random at times. In the end they don’t sound bad per se.
However, I am not a fan of the drums. The kick is barely audible, and the snare sample is quite odd sounding. Their composition though is good, so just changing the samples and improving the mix would do wonders. In general, improving the mixing of the track would help it quite a lot.
A lot of these I get are hard to (if not impossible) to improve as garage band is (certainly was), well, rudimentary to put it simply :p However the one thing that garage band does not have an effect on (composition/arrangement) is in order, which is very good.
I would suggest you take a look at some other DAWs (Digital Audio Workstations) that exist out there. A number of them are completely free, LMMS comes to mind for example (not sure if that ran on mac, which I assume you are using). I tried garage band fairly long time ago, but at least back then it was lacking features (custom audio samples, custom audio plugins) that are basically standard in other DAWs. Not sure if garage band has caught up, but just little something to think about.