Before buying anything you should first make sure your machine will accept the upgrade you have in mind.
Minimum specs for Snow Leopard. Mac OS X v10.6 Snow Leopard -Read the Technical Specifications
From the USA go to http://store.apple.com/us/product/MC573/mac-os-x-106-snow-leopard
In Europe or otherwise substitute your national code in place of /us above.
In the UK … the last known price was £14.
Before you start upgrading, back up your files.
Once you have upgraded to Snow Leopard you will need to apply the Combo updater. Mac OS X 10.6.8 Update Combo v1.1 and follow with a software update to pick up any later improvements.
Upgrading past Snow Leopard. For Mountain Lion, minimum specs … technical specification
Start by buying the Snow Leopard retail disc 10.6.3, then update to 10.6.8 via the combo updater and run software update.
You can go directly from Snow Leopard to Mountain Lion via the App Store, providing you meet the specs.