Itunes ipod

I have an iPod Classic that has quite a lot of music on it.

It used to be synced to iTunes on my iMac.

Recently however, I had to wipe out my iMac O/S and start from scratch. Presently, my iMac has High Sierra on it.

When I fired up iTunes, of course, it had no music in its library.

I don't want to lose all the music I have on my iPod, so what steps should I take to ensure that my iTunes library on my iMac gets ppopulated by whats on my iPod, rather than iTunes completely erasing everything I have on my iPod?

Thank-you for your time and expertise!