When I start to compose a piece of music, I find that I just need to put something on the page or on tape. I might start with a rhythm, or a chord progression, or a few notes of a melody and mess around with it. I also sometimes start with a concept in mind. For example, if I have a scene for a film I need to score and I have an emotion I want to express, I start with the emotion and work out what might express that. Or if I have a piece for marching band I am arranging, I start with the instrumentation I need and work out from there. The point is to start with just about anything and play with it until you have something you like (or works). This can be applied in just about any other genre or medium. If you are painting, put some color on the canvas and see what you can do with it. If you are creating a video, shoot some scenes and see what it looks like and work from there. If you are writing, put some words on the page and see what comes of it.
What do you think?