HT1338: OS X: Updating OS X and Mac App Store appsLearn about OS X: Updating OS X and Mac App Store apps
Currently Being ModeratedJan 23, 2013 4:21 PM (in response to dearcher)
Choose About this Mac from the Apple menu, verify that your Mac has at least a Core 2 Duo(not Core Duo) CPU and at least 2GB of RAM, phone the online Apple Store, and order a download code for Lion.
If you want, open the Mac App Store and try purchasing Mac OS X 10.8. It won't allow the purchase on a configuration which can't use it.