Neblix already covered melodies rather well, so I'll focus on instrumentation and mixing. There's also rhythms, but I'll let someone else cover that.
Let's use Prisoners for this example.
0:00 Right from the start, the drums stand out in a bad way. This is when you need to figure out what kind of sound you want, and design that sound. Drums are often layered, so that eg two snares with different good qualities are combined into the same snare sound, maybe one with a nice strong attack and the other with a nice long body and tail. They are also processed with different effects to make sure they have their own place in the mix... that they're the right amount of loud at the right time at the right frequencies.
0:04 Once the chords come in, the bells are probably too loud and clutter up the soundscape with their shrill sound. I would bring down their levels quite a bit there, and again when the piano comes in, if I wouldn't remove them entirely at that point in favor of a cleaner mix. I could also combine the bell melody with the slower melody that comes in with the chords, to cover that range and bring a bit more rhythm to the track, but that's not mix/sound design.
0:13 The piano is rather loud and rather exposed, and has an annoying reverb. I would soften its reverb. I'd adjust the piano's sound to give it a little more punch, just so I could play it at a lower level and still have it get through enough.
0:30 The bass is panned. Don't do that. Most of the time, the bass works best centered, for both technical and aesthetic reasons. I won't go into the bass' rhythm or melody. It plays on top of the kick drum, and has a similar kind of sound. That doesn't work, because I can't hear the kick anymore after the bass started playing. For some genres, you'd want the kick to peak at a lower frequency than the bass, for some genres vice versa, but you don't want them to fight over the same frequency. This is where you change one of them so it has different qualities, eg stronger sub, punch at a different frequency, brighter overall sound... The synths/samples you use affect this the most, so adjusting those is the best course of action here. Other solutions include transposing the bass, and separating them with an equalizer.
0:48 The trance synth doesn't quite fit the aesthetic. There's also a ton of stuff happening in the track right now, making it a mess to listen to, especially as there is no clear lead. This is both a mixing and an arrangement issue. The mixing issue is best solved by picking a track to be the lead, deciding a frequency range that it's got a clear sound in, and cutting down that frequency a little from the other instruments. It's good to do, but it's not a replacement for solving problems in the arrangement.
My guess is that you're using a lot of presets without modifying them much. timaeus already suggesting learning to make your own sounds. Even if you go back to presets after that, you'll know how to adjust those presets to better fit the rest of the instrumentation. Beyond fiddling with the instruments themselves, you should learn how to use effects. the basic ones, the ones everyone into this should know the basics of, are equalizer, compressor, reverb; and most importantly and often neglected: track level (volume).
This track isn't that different than the stuff I made when I was 16, except you've turned to people for criticism much earlier than I did, and are taking music courses in college. Learn to identify things you're not happy with, and find solutions to those things. That's how you learn.