Software Update

I guys,

I am trying to update my MacBook3,1 from Mac OS X 10.5.8

But when I use the software update, its telling me that there is no new update...

I am a bit confused

I just got a Iphone 5 but cant plug it on the Mac coz i need at least Itunes 10.7 but for that my Mac OS X is too old.

Any help please?

Cheers