Erin, in the middle of kindly linking here, says she can't imagine how you'd go about composing. Well, it's easy! Here's the Defeatist's Ten-Point Guide to Composing:
1. Take your instrument. If you don't have one, just sing or hum or something.
2. Play (or sing or hum) a tone. Carry on as long as you feel like it (or can without asphyxiating)
3. Now do another one.
4. And another.
5. Maybe you fancy playing a bunch of fast ones? Or not? Well, do or don't, whatever you feel like.
6. Maybe you fancy playing more than one at the same time? Well, why not? If you can only produce one tone at a time, consider having a friend handy.
7. Continue in this vein as long as you please.
8. Hey presto! You've just composed!
9. Write it down if you like. Or not. Doesn't really matter.
10. If you want to do something more complicated there'll probably be editing involved, but that's about it, really.