how do i update my software i currently have a Macbook 10.5.8

i noticed i had an older version of the Mac when i tryed to download a Photoshop Free Trial off of the Adobe website.. and noticed they said my version was to low and i could not download it.. i was very shocked because tons of people i know were able to do it.. and they didnt even have a mac..
if someone can help me find a way do upgrade my software.. how do i do that.. where.. and is it possible to do online..
Thank you.
( the photoshop required a min of Mac 10.6 )