Linda, you need to get a cable to connect the output of your keyboard to the input of your sound card on your computer so you can simply record your keyboard as it plays a MIDI file.

You just need a cable that connects to the the stereo output of your keyboard and an adapter to allow it to plugin into the 1/8 inch stereo input on your PC. Most electronics stores should be able to provide you with these. Then you just run any audio recording software and press play on the keyboard and then save the resulting wav file when it has finished. You can then make an MP3 file from that wav file.