First Check that your Mac meets the System Requirements for SL and ML...
Snow Leopard Tech Specs
http://support.apple.com/kb/SP575
Upgrade to Mountain Lion
If so Contact Apple in your Country to Purchase a Snow Leopard Disc...
http://support.apple.com/kb/HE57
In the US...
1-800-MY-APPLE or 1-800-676-2775
Be sure to make a Backup of your Current System Before Upgrading...
After the Successful Install, run Software Update to get the latest updates for Snow Leopard.
Next go to the Mac App Store and Purchase Mountain Lion...
Upgrade to Mountain Lion
You may wish to add more RAM...
It is important to get the Correct and Matching RAM
See Here > OWC RAM > http://www.macsales.com/
Lastly... with regards to Mountain Lion...
Check here for compatibility of 3rd party Software you may be using...
http://roaringapps.com/apps:table