You can use Genius to have iTunes play songs from your library that sound great together. There are three ways you can use Genius:
Genius Shuffle: If you can’t decide what to listen to, just click a button. iTunes chooses a song and then plays songs that go well with it.
Genius Playlists: Choose a song, and iTunes creates a playlist of songs that sound great together.
Genius Mixes: Choose a genre, and iTunes plays selections of songs that play continually, like commercial-free radio stations.
Genius is a free service. To use Genius, you must turn on Genius (on each computer you want to use it on), be connected to the Internet, and have an Apple ID.
If iCloud Music Library is turned on, you can’t turn Genius off.
Choose File > Library > Turn On Genius (or Turn Off Genius).
Turning off Genius turns off Genius Playlists, Genius Shuffle, and Genius Mixes.
Genius Playlists appear in the sidebar on the left, below Music Playlists. A Genius Playlist has the Genius icon 
next to it.
Create a Genius Playlist: Select a song, then choose File > New > Genius Playlist.
Change the maximum number of songs in a Genius Playlist: Click 
next to the song total, then choose a number from the pop-up menu.
Get new songs based on the same song: Click Refresh.
Hold down the Option key, then click 
near the top left of the iTunes window. You can also choose Controls > Genius Shuffle.
To change the selection of songs, repeat the above step.
Choose Music from the pop-up menu at the top left.
Select Genius Mixes in the sidebar on the left.
To start playing a Genius Mix, move the pointer over the mix you want to play, then click the Play button 
.
Your Genius results are automatically updated once a week when iTunes is open. You can update them manually—for example, when you add new music to your library from a CD or a website.
Choose File > Library > Update Genius.
To create Genius Playlists, Genius Shuffle, and Genius Mixes, iTunes uses anonymous information about your library and other iTunes libraries. The iTunes Store updates its Genius data constantly, based on new information from the iTunes Store and customers’ libraries.