First, install OS X 10.6. You can only buy this as a mail-order disc (cannot be downloaded via App store). Once installed, take all updates to get to 10.6.8.
If you only want to use the latest iTunes, stop here – that’s all you need.
And if you want the latest OS X 10.9, get that from the App Store for free, but make sure you have the hardware requirements before continuing. Note that you can’t even find 10.7 or 10.8 anymore because they’ve been replaced by 10.9.
