authorize a new computer

If it is a new computer, use the Music app. (If it is an older machine, use iTunes.) Open and go to the Account menu, Authorizations, and then "Authorize this computer." using your Apple ID and password.

Once you are set up, you can download past purchases per this guide:

Redownload apps, music, movies, TV shows, and books - Apple Support

I can't reinstall OS to new Mac. How do I authorize new computer after deleting old OS