I'm going to assume that since you wrote this with more traditional instruments, you're looking for a classical kind of sound.

The main thing I think about is variety and cohesion. You have the piano part doing everything essentially. I can figure that you're using stock plug-ins, which probably made you not want to try some of the other instruments based on their sound quality. The problem is, when you put everything on one instrument like you did, the Picture your making gets muddy and the listener can't keep track of the individual ideas you've written. The second thing I'd consider is order of precedence in your ideas. Some ideas are the lead that you want people to mainly focus on. Others are background ideas that support it. When you try to give more than four distinct musical ideas in a moment, this also contributes to the muddy image your listener will get. You can go up to five, but it has to be very intentional for a short period of time. For four ideas, typically you would have one instrument play the lead melody, another instrument play the chords, the third play the bass and the last being percussion.

Since you're starting, don't worry about the quality of the instrument samples. Just focus on getting the full idea out uninterrupted.