I'm not a MAC person, but assume that iTunes works the same as on a PC.
Make a new playlist in iTunes.
Drag songs from your main library to the playlist.
Open the playlist and sort the songs to your liking.
In the lower right hand corner is a button called "Burn CD".
Insert a blank CD, click the button and wait a few minutes.
(If your car CD players will play mp3 files, you can fit over 100 on a CD. With regular music files, you can fit 15-20.)
Good luck on your first burn!