You've got to upload your mp3 to an mp3 host somewhere - a lot of people here have used Nowhereradio.com. Your mp3 sits on the host's server, and can be downloaded or played anytime anyone wants to.
Then you put a link to your song in a post here, probably in this clinic. People click on the link and it takes them to the page that's got your mp3(s).
The internet's a wonderful place.