Well, any time I've ever made a song, I was just messing around on the piano. I didn't write sheet music for it, and I usually forgot it by the next day. Generally, I'd follow a slower tempo and major keys. But, when I started making this song on the computer, I found myself sticking with minor keys, and a fast tempo. As I kept going, it ended up sounding very classical rather than a nice, pretty contemporary piece.
Though, I haven't actually written a full song before (except for this one). All the other times it was just intros to songs that I'd forget the next day anyway.