IPhone/iTunes Music Sync

I want to sync songs off of my iPhone onto my iTunes but every time I try I get scared because a window pops up saying:
"Are you sure you want to sync music? All existing songs and
playlists on the iPhone "myiphone'sname" will be replaced with
songs and playlists from your iTunes library."
If that happened then everything on my iPhone would be erased and I do NOT want that to happen!
HOW can I get my songs onto my iTunes without anything being deleted?

If you click sync then a box should pop up saying transfer data. That will just get the stuff on your phone onto your computer. If not then something wrong with your itunes and try updating it if its not up to date