First of all format the USB stick to FAT32 format which can be read by pretty much any USB-equipped car stereo head unit. Then I find it best to make some Genre folders at the top level. Something like:
or whatever takes your fancy really. Then within each of those folders, copy a folder for each Artist you want to have. Inside each Artist folder should be .MP3 files and nothing else. Strip out all the album art and other hidden system files.
On my VW Golf's stereo, I can browse the folder list and choose the Genre I want to play e.g. Latest Rock/Pop. It will play all folders in there and then move to the next Genre folder.
Your stereo should have the ability to display Track, Artist, Album or file name. My preference is to see each track showing ARTIST - TRACK NAME while it is playing, so I set my stereo to display the file name, and have organised my library to use file names in the above format. When ripping CDs you have a choice of how you want the file name to be composed, so that is the format I chose. If you only want to see the Artist or Track Name it is easier because the stereo can do that by itself.