Hey dgoto,
Open the Music app while holding down the Option key. You'll get a little pop-up asking you to choose a library or create a new one. Select the new-library option. The Music app should now display a brand-new greeting page and a clean library.
Now there are a number of ways to import your music collection. My method of choice would be to drag the whole media or music folder (wherever the files are physically located) into the folder called Automatically Add to Music. You can also just drag the folder or folders into an open Music app window, or select "Import..." from the File menu.
Before you do that, have a look in Music/Preferences/Files and adjust those settings to determine where and how your new library will be set up.
Cheers