I would buy OS X 10.6 from the Apple store, install it, run the software updates, and then buy OS X 10.8 (the latest version of OS X available). Apple has announced a new version of OS X (Mavericks) to be released this fall.
Head to the Apple menu > About this Mac > More information and give us the model identifier.
It looks like this "MacBookPro 4,1" also the RAM amount and your OS X version (like OS X 10.4 or 10.5)
From that information we can give you a idea of what or what not to do.
Except in the case of a Mac Pro tower, any Mac with 10.4 or 10.5 is typically too old to run the latest operating systems of 10.7 or 10.8 well, and since 10.9 is coming soon as well to deny even more older machines a OS X upgrade.
However OS X 10.6 is still getting them as 25% of Mac users is on it (not Safari or Java 6, use Firefox instead), so it's possible you may be able to upgrade to that and it allows you to still use the software on the machine, unlike 10.7 or later which won't use the older PPC binaries.