Currently Being ModeratedFeb 9, 2013 9:45 PM (in response to donald242)
If you have a G3, it can't be upgraded further.
If you have a G4 or G5, buy a retail Mac OS X 10.5 DVD from a source such as Amazon or eBay.
If you have an Intel Mac, buy a Mac OS X 10.6 DVD from the online Apple Store.
If you don't know, choose About this Mac from the Apple menu.