I was just reading about Songsmith, a new Microsoft Research invention that can help you with composition of your songs. Yes, you got me: you start by singing your tune - whatever is in your head - and the software than adds up all the music backup needed. You can choose different music styles (like rock, etc) and voila you have your No.1 single :). Go check it out, I am downloading it now: http://research.microsoft.com/en-us/um/redmond/projects/songsmith/.