Hi Larry,
Before you can create an MP3, you need to create a WAV file of your song in your PC. I recorded my keyboard & vocal performance directly into my computer's hard disk using Emagic Logic Audio (Digital Audio Sequencing program) via a computer sound card (Midi-man Delta 66). You can use the generic soundcard that came with your PC, but you will achieve far better results with a high quality 24 bit audio sound card made specifically for recording digital audio. These have much cleaner (quieter) A/D converters and better sound. I used Logic Audio to create a WAV file. From there you can easily convert the WAV file to an MP3 using an MP3 encoder software program. Here the decision is more difficult as there are many Mp3 encoders (both free and shareware) available. Here is a link to a website that provides the lowdown on MP3 conversion/encoding: http://www.mp3-converter.com/encoders/

Hope this helps, - Scott