Help me upgrade my iMac

Got a iMac and it's had the same software since i got it about 6yrs ago so it's really slow and most things say the browser doesn't support it or software doesn't support it but i think i have to do more than one upgrade to get it up to date?

It has Mac OS X version 10.5.8 which i believe is Leopard. Do i have to upgrade to snow leopard then Lion then mountain Lion or is there an easy way to do it?

Thanks