I use a midi controller and just wrote a really good song, I used all the layers of piano and strings etc. How do I burn it to audio to listen to in my car and put online?
Two methods depending on the source of the midi sounds.
If you used soft synths your software should have a 'render to disk' option which will create a stereo .wav or .aiff file which can then be burned to CD.
If you used sounds from a hardware keyboard or module you need to record the sounds onto audio tracks in your sequencer, then follow the procedure above.