What are you trying to accomplish?
If just backing up music to USB, all of the media should be in ~/username/Music/iTunes/iTunes Media/ if your library is organized. For full backup, capture everything under/Music/iTunes.
If you are trying to put files on a USB stick to play in your car, etc, you may need to convert files to MP3 or other compatible formats. In iTunes, select the song(s) you want to convert and choose File>Convert. This will create a duplicate file in the existing file structure, then copy it to the stick. Some MP3 players will read the folder structure, so you could create a folder with artist name, then another with album name then drop the MP3 in there.