Installing lion over leopard

I have OS X version 10.5.8 and since it isn't being supported by most companies anymore (ex. Blizzard) I wanted to upgrade to Lion. I then got a Lion USB because I read that I could download from 10.5.8 directly to Lion. However when I inserted the USB, my computer told me I couldn't download it on this version of OS X. I meet all of Lion's system requirements (excepting having snow leopard installed). So why can't I install it? Can someone give me step by step help?

 

I did some more research on this matter and I found that I need my installation disk (to wipe the hard drive and install Lion), however over the course of 5 years I've lost it =|

I tried inserting a Leopard installation disk so I but I get the message... "The application 'Install Mac OS X' cannot be used from this volume. To install Mac OS X, please use the application provided on the Mac OS X installation disc."

 

thanks for the advice =D

I guess I'll just search some more before I end up ordering a new one