An early 2008 MacBook cannot be upgraded to OS X 10.8 Mountain Lion. If you have 2GB of memory (4GB recommended) you can upgrade to OS X 10.7 Lion, but that's as far as you can go on that system.
First, you have to buy operating system upgrades, so call the online Apple store, which in the US is at 800-MY-APPLE, or order OS X 10.6 online at http://store.apple.com/us/product/MC573/mac-os-x-106-snow-leopard . Install it, do Software Updates, which will get you to OS X 10.6.8. Then call the online Apple store to buy a download code, which they will email to you. Then use the App Store to download OS X 10.7 Lion.