Do you listen to lots of musical scores, soundtracks, etc?
I am very interested in composing and that is what got me started.
This should help you hear tunes in your head. Think of a cool melody for a long time that sticks in your head. Then arrange it for piano.
Also, try your best to compose music in your head. If your good at this and you know some theory and your notes on the piano well, you should have no problem!
being a good sightreader helps, too. (if your no good at sightreading, then sightread!!! play through any music (not too hard) that you can get your hands on. play thru it without stopping!)
-t-rex