Step 1: Delete the duplicates
You may have duplicated songs you want to delete. This cannot be done in iTunes 11, so in previous versions, go to File>Show Duplicates and delete. Depending on your version you may find this particular option under the View menu.
Step 2: Get Match’d
To rid your Mac of music, you may want to consider iTunes Match, which allows you to stream and sync your library through the cloud.
Step 3: Remove songs
If you’re the kind of person who likes to rate songs, you can remove unwanted tunes pretty easily. Select Music>Songs. Click Ratings to group one and two-star ratings. Hold Ctrl+Select+Delete.
Step 4: Consolidate files
Go to File>Library>Organize Library, select Consolidate files and hit OK to consolidate all types of media into one easy-to-find place.
Step 5: Create sub-folders
You are also able to create sub-folders and place imported media from Music to Audiobooks into them. Just tick the Reorganize files box.
Step 6: Edit the metadata
Ensure that the metadata is working well for you and has, at the very least, correct info. Right-click a song, select Get Info and choose the Info tab.
Step 7: Batch edit music
You may need to edit a lot of similar tunes. Maybe a band’s name is misspelled. Highlight a batch by Ctrl+clicking tracks. Right-click and select Get Info.
Step 8: Grab album artwork
If you want to find missing album artwork for songs in your library, click File>Library and select Get Album Artwork. Your iTunes app will now perform a scan.
Step 9: Sweep through tunes
Sort your songs by name and look for any that are called unknown or untitled. Do the same with artist and album. Ensure you alter the metadata too.
(Click on the image below to zoom in and view the annotations.)