First you need to research all the problems people are having with the higher OSX versions, & make sure you have a bootable clone of what you have just in case.
then you must get 10.6 if they still have it, install it & update to 10.6.8 so you have the App Store to buy & download the huge 10.8 Installer.
Snow Leopard/10.6.x Requirements...
* Mac computer with an Intel processor
* 1GB of memory (I say 4GB at least, more if you can afford it)
* 5GB of available disk space (I say 30GB at least)
* DVD drive for installation
* Some features require a compatible Internet service provider; fees may apply.
* Some features require Apple’s MobileMe service; fees and terms apply.
Which apps work with Mac OS X 10.6?...
It's been pulled from the online store & Apple Stores, so you have to call Apple to buy it, last I heard.
Call Apple Sales...in the US: 1-800-MY-APPLE. Or Support... 1-800-275-2273