first you want to decide on the genre. Mine is easy... it's mainly reggae
if you download Winamp for example, you can tune in to loads of Shoutcast Livestream internet radio stations. They are ordered by style, so a good one is found easily. Tune in! you might hear remarkable things which you might want to try yourself!
You also might want to select a livestream from a genre you didn't have in mind to track in, but it might provide you with some fresh ideas for your original tune.
I also often start with chord experiments. I go to the Instrument Editor and press some random chords. I have done this a lot in the past, so I can predict pretty well what sounds right and what not, but that's a matter of experience.
Once I made up my mind, I start tracking the Chorus. I've tried to start with an intro some times, but the possibilities how to continue overwhelmed me, read, blocked me. But that approach might work for others.
When you have a chorus, you can work around and towards it.