When I go to my Partition tab, there is no blue section, and it will not let me select anything, and says I cannot erase my startup disc. Other suggestions?

I'm trying to download Mavericks on my 2007 imac version 10.6.8. When I attempted to, it said "Install Failed: OS X could not be installed on your computer. OS X can't be installed on the disk because a recovery system can't be created. Visit www.apple.com/support/no-recovery to learn more" I've tried other discussions and they say you must erase your disc, but I want to make sure I have a back up of version 10.6.8 first. I'm not sure where my CD ROMs are that came with the computer to download the start up disc. This forum suggested changing the partition manually in the Disc Utility, but when I went there, there was no partition to change. Any thoughts? Do I just have to hunt down my start up disc download? Is there a way to download Disc Recovery???