Musicians: How can I make a great song?

I've been playing piano and a few instruments for a while now. But at times,depending on how I feel, my mind starts to think of a song I make up while pretty much creating an orchestra in my head where I can think every arrangement for a singer, instrumentalist, time signature, key of the song etc. So my question is, How do I try and put what I'm thinking in my head to sound good on a sound track etc. Will I need to use a program like acid, fruity loops or something? I've already put down what I think on a keyboard with separate tracks, then combine everything to one. And I know if I do this right, this can be a hit song. I don't sing either, but I can pick out amazing melodies a singer can sing.