How do I sync my iphone music with my itunes library? It says my itunes is empty and if I sync it will remove all my music Please help!

Hi all

I'm trying to sync my iphone with my itunes library but it says my itunes is empty and if I sync it will remove all my music Please help!

The music is stored on my old computer too (which has gone to computer heaven!) So if it gets removed I'll have a little cry!

I would also then like to share this music with my ipod touch 4th gen. running ios 6.1.6.

Thanks everyone.