You can play music from iTunes on the Clip (as long as it is not DRM protected). However, the Clip does not play aac files, the iTunes default; you need to convert your iTunes aac files to mp3 files (or another format supported by the Clip). iTunes provides instructions under its help menu, under exporting.
You also can rip what you might have on a CD for use on the Clip; again, just rip into a format supported by the Clip.
Once you have your music in a format supported by the Clip, you easily can simply drag and drop the files onto the Clip (they can be dropped anywhere on the Clip, although the Clip’s Music folder is a good place to drop them, to help keep things organized). Or you can use third-party software to transfer the music–Windows Media Player, Winamp, Media Monkey, etc.