I have a new Mac and all my music is currently stored on my phone, if i sync my phone to my new mac it says that all my music will be lost. How do I prevent this?

I have a new mac computer so my itunes library is empty. i tried syncing my iphone with this new library and it keeps giving me a pop up window saying that all my content on my phone will be replaced with the content on my ocmputers library. My computers library is empty! why would I want to do that? I would like to keep all my music but I cant figure out how! please help me.