How to sync my old itunes library to a new library without erasing my music?

Okay, heres the story. My itunes library for my iphone was on a computer which we recently got rid of.So, I made a new Itunes on my laptop and I need to sync my music that's on my phone to my new itunes library. I would like to do this without erasing my music cause there's no way I could get it back. If you know how to do this Please Help ME!

I would have backed up you music to an external from you iphone. I know ITunes libraries like to be synced. If in doubt ring Apple Support, it may cost if your apple care has expired but the helped me sync iTunes from a PC laptop to an iPod.