Recommended application for it: CDBurnerXP (Freeware). http://cdburnerxp.se/
If you know the good old Ahead NERO 6, you'll be glad to have it. I never used any other program to burn something (Except ImgBURN for cd/dvd-images).
If your car supports the ability to play mp3-cds, simply burn your mp3 as "data". If not, choose "audio-cd" (your mp3-files will be automatically converted).
It's easy: Just Drag & Drop the files you want, a bar at the bottom of the application will show you, how many free space you have & if your file(s) will fit to it.