Good question. And a bit beyond the scope of this instructable.
For me, I perform my songs live, record them, make CDs, give them away or sell them at my shows or in local stores. And no, I haven't quit my day job.
(My music pages are here; and here, videos are here, free mp3 downloads of everything I've ever recorded is here)
Another thing to do is try to market your songs. Good luck with that; it's a lot of work... but you'll get a paycheck now and then if somebody records it. If you don't play, look around for players willing to give your song a try. Hit your local open mic. Join a songwriting contest. Sing around the campfire.
But regardless of what you do with your song, work on it a little. Do a great job on it. Think about what music has meant to you; that one song that really hit you hard when you heard it on the radio or that movie soundtrack... why not shoot for that? Even if the world doesn't get to hear it, you'll know that you did good work, and that's fulfilling enough to be well worth the attempt. Right?