How do I download my itunes onto a mp3 file to run in my car?

Presuming they aren't mp3 format files:

iTunes: How to convert a song to a different file format - http://support.apple.com/kb/HT1550 - including information about different formats and discussion about compression.

In iTunes11: File > Create New Version

In iTunes 11 "Create..." is in the File menu, not the Advanced menu.

Topic: Need to convert mp4 to mp3 - Zevoneer's post on converting songs - http://discussions.apple.com/message.jspa?messageID=9244740

Converting will place duplicates in iTunes but if you look at the kind you will see one version is mp3. Drag those files to the USB drive.

In iTunes 12.3.2, you need to change the "Import Settings" (iTunes > Preferences>General) for "When a CD is inserted:"

Select "MP3 Encoder" next to "Import Using" and select OK

Select on your iTunes song, right click and select "File>Create New Version>Create MP3 Version" and the song gets converted to MP3

To double check song file format, select the duplicate song and right click song - Get Info>File to verify MPEG-1, Layer 3 format.