how can I upgrade my computer to Mountain lion if it's only at OS X 10.5.8?

I'm trying to upgrade my computer from leopard to Mountain Lion and I'm having trouble figuring out how to download. My systems are to out of date in oreder to download it directly. Has anyone had this same problem in the past? if so, how did you solve it? If I go to the apple store and buy the disc will it be easier to install? or is there a way I can work around the problems from home?