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The article below will show you how to send a song from Garageband to iTunes. You can enter song information in the Share dialog box.
Also from within iTunes, you can edit song information as well. Here's how:
Each audio or video file in iTunes has several settings and tags that can be easily modified. These options can be accessed by selecting any file in the iTunes browser windows and selecting "Get Info" from the File menu. You can add song lyrics, artwork and set special playback options. The "Info" tab lets you edit information about the file including artist and album info.