Genius makes playlists from songs in your library that go great together. The Genius sidebar recommends music you do not already have from the iTunes Store.
Launch iTunes by selecting its icon in the dock.
Notice the new Genius bar at the right of the iTunes window. If you do not see it you can click on the Genius sidebar toggle button.
Click the Turn on Genius button in the sidebar
Click the Turn on Genius button again to continue.
Enter your Apple ID and password then click the Continue button. If you do not have an Apple ID you can sign up for one here.
Check the box to agree to the Genius Terms and Conditions and then click the Continue button.
iTunes will now gather information about your iTunes Library.
It will then send the information to Apple.
You will now be informed that Genius has been turned on. You can now create Genius playlists and use the Genius sidebar.
Notice that if you select a song from your iTunes music library the Genius sidebar will populate with Top Albums, Top Songs You Are Missing, and Recommendations
Select a song from your iTunes library and click the Genius button.
iTunes will now populate a list of similar songs from your library and create a new Genius playlist.
You can limit the Genius playlist to 25, 50, 75, or 100 songs, refresh the playlist or save the playlist.
Notice if you click a different song in the playlist the Genius sidebar will again populate with Apple's recommendations