Ahh that's it.
When you feel that you mastered this program, try others. Or change the music samples (sounds for your music)
Ok you should listen some of the music genre you want to compose and start thinking about the instruments they use. What instrument should have more volume and what other should have less and vice versa. You have the melody on your head, just keep practicing with different sound programs. Think about the tempo too, that helps a lot in the composition, the: one two three four.... one two three four. oneeee two three four.
Man. I want to read theory of music again.