Depends what you mean by 'write'. If you are a writer who can sing and play an instrument adequately but does not perform, you can record a demo, turn it into an MP3 file and post it for free on the internet at sites like soundclick.com and ic-musicmedia.com. They give you a free artist page and, hey presto, you have a shop window. Selling your song or even giving it away is of course harder, but this way at least the song exists for someone else to hear.

If the song only exists in your head you can't expect anyone else to download it!

I started playing in public after 15 years of non-performing. It's helped my songwriting a lot.