Seems to me you need to first .....
Thoroughly check the details in the link sanpamjet gave you. I'll repeat it.
If your machine qualifies, bring it up to the maximum level for the OS you use. That is ...
Then follow up with software updates to pull in the iTunes and security updates. This should set you up for further progress.
All being well, you can then go directly from Snow Leopard to Mountain Lion via the App Store, providing you meet the specs.
MAVERICKS Tech Specs Apple - OS X Mavericks - Read the technical specifications.
Be aware that although Apple suggest 2GB RAM, the general concensus here is that 4GB is needed. And that any PPC apps you have will be rendered useless by the update.