I used to wonder the same thing, then I realised I didn't have to be a composer just because I liked contemporary music. It was a very liberating discovery for me (since it allowed me to do other things, such as write about the music, without counting them as peripheral activities or sidelines), but I wouldn't give the same advice to everyone. And what I probably leave out of account is how the attempts to compose, however insignificant the results, informed my understanding of how other people do it.

How do you compose? As my old Professor of Music would have said: 'The first note is easy. It's the second note that's difficult.'

Erm ... that's not very helpful really, is it. Makes sense when you've tried though.

That's like the now-famous Birtwistle answer to the question, 'where do you start?'
'Bloody anywhere.'

Which is a liberating idea, but no more helpful than the old Professor...