Before sharing your library, you need to know that the iTunes application needs to be turned on. Open iTunes and go to Preferences (Mac: iTunes > Preferences , Windows: CTRL + B > Edit > Preferences). Go to the Sharing tab and select Share my library on my local network and that's it.
There is a more in-depth guide about this procedure. You can access it using the following link: https://support.apple.com/en-us/HT201779