Firstly (as you probably know) you have to convert your mp3 into a streamable file.
The most common one is RealAudio by real.com, I believe you can download a demo of RealProducer which allows you to covert the mp3 file into the streamed realaudio file. Another popular streaming tool is Windows Media Encoder by Microsoft. I remember that RealProducer includes a set of wizards to help you convert the file into its streaming format. I can't remember if Windows Media Encoder has one, but being Microsoft it probably does.
Now if you use RealProducer it will covert your mp3 file into a .rm file (if I remember correctly, it has been a while). Next you open up a text file (try Notepad) and copy into it the url of the .rm file, such as: