So how do I save everything on my Ipod touch to iTunes before updating it??

So I have an Ipod touch 4th gener I believe.. So I need to save everything so it can be updated, ok, So it tells me to, Transfer Purchases from Ipod, ok, So I do that and it says it's not Authorized, so I do everything it tells me about authorizing it so I can update it safely,then get their to find out it is Authorized. SOO can someone please help me by telling me or etc on how to save everything from my Ipod So it can be updated without erasing anything. well Thank ya'll