Here's a free program you can use to write your Incredible Compositions for posterity. If you have used Sibelius at school, this program should feel familiar. When you have written something, you can press a 'play' button so you can actually hear what you've done. It's fun. Ask me if you have any questions regarding program installation, how to actually write the music in the program - anything you need.
I had a go at this program a couple of months ago, and it was pretty easy to get the hang of. You can see the tasteful results of my efforts at the beginning of this post.