outdated computer buy about 7 months and will not find any new software updates when I check

I have a macbook os x 10.5.8. I just bought a new iphone 5 and need to have the latest versions of mac os x and itunes but when I check for updates on my computer nothing will come up, although I know there are several newer versions after 10.5.8. I tried downloading the newest mac os x version but of course I needed the previous version to download it. Need to download 10.6. Where can I find it?