HT1338: OS X: Updating OS X and Mac App Store appsLearn about OS X: Updating OS X and Mac App Store apps
Currently Being ModeratedJun 4, 2013 2:54 PM (in response to lecco19)
Choose About this Mac from the Apple menu and check the processor.
If it's a Core Duo Mac, click here, install the DVD, and run Software Update.
If it's a Core 2 Duo or better Mac, follow the Core Duo instructions, open the Mac App Store, and try purchasing Mountain Lion. If you get told it's incompatible, phone the online Apple Store and order a download code for Lion; a regular MacBook(not MacBook Pro) which shipped with 10.4.11 or earlier is incapable of running 10.8.