My current iPhone is ran by my desktop pc (like all the contacts, messages etc) but that iTunes has no songs on it. Yet my brother who has laptop, his iTunes has loads on. Can I sync music from his iTunes to my iPhone without losing my contacts etc? Because when I press sync music it says something like all contents on this iPhone will be erased, is that literally everything or just the shitty music I've got on my phone at the minute?

Its everything, you can only link your iPhone to one pc. Get a usb stick and copy the music over to your pc

Your itunes will carry a backup of your phone, but just as suggested get his music library and import it to your itunes.