Live doesn't support exporting your song as an MP3. There are however, many free tools to convert a WAV or AIFF file into an MP3.
For OS X we recommend iTunes. A tutorial about how to convert audio files into MP3 with iTunes can be found in the Apple knowledge base. iTunes is available for Windows as well.
Another good tool on Mac is Max (not to be confused with Cycling 74's Max/MSP).
LAME is an MP3 encoding resource, used by many freeware programs, a good example of which is LAME Front End, which provides many easy-to-use encoding options.
Audacity is also available for both platforms, but you'll need to follow some instructions to get it to export files as MP3 first.
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