I've been given a similar task over half term (also my first piece of composition hwk). We have to write a 3 minute piece of film music, having been given the title 'Darkness to light'.
I really didn't have a clue where to start, so just decided piano solo would be easiet, c minor was a good 'dark' key and sat at the piano and tried to come up with some kind of chord progression because that was the bit that scared me least. Once i'd got something - only 4 bars, I just tried playing anything above it to try and find a melody that worked. But then I realised I wanted a bass part, but had run out of hands, so I added a cello, then I thought the chords were a bit bland so the piano got bigger chords and a violin got the melody. And now i've pretty much got the dark bit done, just have to find a not-too-cheesy way to go light.
I think rather than decide in advance what instrument/melody etc you're going to use, work out either the harmony or melody, and let the rest come from there.
I think that the most useful thing that my music teacher has said to us over last month is not to worry about getting composition wrong, because there is no such thing