Mac OS X 10.5.8 (9L31a) I have this operating system and would like to upgrade...How would I go about upgrading and what system do I upgrade to???

Mac OS X 10.5.8 (9L31a) I have this operating system and would like to upgrade...How would I go about upgrading and what system do I upgrade to And how would I go about upgrading my system? Eventually I would like to get to the Mountain Lion system but I just am not even sure where to begin. Any help, suggestions or what I need to do would be very helpful. Thanks in advance!