Backing up iPhone 6s on PC for the first time?

[iMore Question]

Apologies for the silly question..

My mom has been using her iPhone 6s for a couple of months, but she never plugged it into the PC before. Now that she wants to add custom ringtones she has come to do this but iTunes it stating set up an new iPhone. If i click that, isn't it going to erase everything on it? How can I get my phone linked to this PC without loosing everything?

Thank for you help on something which is probably very simple

 

[Tartarus]

Don't worry, your iTunes is gonna treat it like a new iPhone but is not gonna erase anything unless you want it to. Make sure you have disabled everything you don't want to be synced to the iPhone if it's not a new iTunes install.