ITUNES WON't SYNC MUSIC TO IPHONE?

I have a mac and i use frostwire to download music, and when i transfer frostwire music on my computer it goes on, but when i go to transfer it into my phone it doesn't go on. Now I've clicked sync and it deleted basically all my music on my phone I'm very angry i need help!

Go to File<Devices<Then Transfer purchases and then you can sync without deleting music.