Don't forget the lyrics guys, I know this is a guitar website but if there is a theme of your song write lyrics to suit and accompany the lyrics around the music, rather then music around lyrics.

In other words, decide if the song is going to be heavy or soft or other then write the music remembering to add in a chorus if you have one, then write the lyrics in to the song.

The more you do it the better you will become.
Good luck.