You will need an FTP program, your website's username and password.
Using the FTP program, log in to your website with your username and password.
Using the FTP program, upload the file into the same directory as your homepage. Note the mp3's filename CAREFULLY. CaPitaLIzaTion MattERs.
if the filename is myFile.mp3, and your website name is example.com, the url would be:
http://www.example.com/myFile.mp3
This is pretty simple and typical stuff...Somebody you know already knows how to do this. It might help if s/he stood by to help you. Ask around...who do you know nearby who knows how to do this? Maybe the kid next door?