you can make a play list.
its recommended that you make a playlist.
or it will copy your entire library
you select the playlist
you will see the list of songs in that playlist in that main window
click on command A to highlight all the songs
drag them to a finder window where you target is and it will copy them
your car stereo might not like protected tracks, aac, or only want mp3
if that is the case you can use iTunes to convert them to mp3s
or you can make a smart playlist to make a playlist of only mp3s
it might put them in directories
its been a while since i did this
most of my music is on iTunes match so it only copied the ones that were on the hard disk
u could do the same thing in finder, but its harder and not as easy as using iTunes