The latest OS is Mountain Lion 10.8.3. Before upgrading make sure your Mac meets the specs:
You will also need at least 4gb of RAM if you want using MLion to be a pleasant experience.
Also, be aware many older PPC apps, along with some other programs will no longer work. They will either need to be upgraded or replaced, so if you rely on any particular apps make sure they are compatible with MLion.
If all that sounds good, you'll need to purchase Snow Leopard from the online Apple Store, install it, run software update to bring it up to v10.6.8. This'll give you access to the Mac App Store to purchase and download MLion.
Ensure you have a full and reliable backup of your system before embarking on the upgrade process. Things shouldn't go wrong, but they can. You need to be able to roll back to pre-upgrade status if there is a problem.