You would first have to render the MIDI to audio, either by inserting software synthesizers and playing the MIDI tracks through those and bouncing the tracks to audio, or recording the MIDI playing through a MIDI synthesizer (like a MIDI keyboard) into an audio track.
Then get an mp3 encoder off the internet to render the .wav files (audio) to mp3. Or you may already have an mp3 encoder on your computer. One of my computers does, the other does not.

This is what I use:  http://www.nch.com.au/wavepad/index.html
 
It runs in full mode for a limited time. You can opt to keep it and run it in demo mode when it times out. Everything I need it to do is functional in it's demo mode...... trim start & ends, normalize and convert to MP3.