How do you unsync an iPhone from a computer?

I want to download a song from a mac but my iPhone is pc based. My friend told me that I only had to unsync my iPhone from my computer and it would work because that's how she said she does it. So how can I transfer a song onto my iPhone with screwing up her computer and my iPhone?|||Connect it to the Mac and set it up with that iTunes. It will want to restore it, so anything that is on there now will be lost and you'll have to do everything through the Mac computer.|||just put the usb in to the computer and iphone and wait for it to recognize the phone (on the mac) and if it dont then it will have u pick a name for the phone then itl be on