Did you try the troubleshooting steps in the last 3 links? To upgrade past 10.6, you need access to the App Store so you can download the OS. If you have access to another Mac, sign into the App Store with your Apple ID and you can download El Capitan. Use the program below to make a bootable USB Flash Drive. You can then boot off the USB and erase the drive, then install the OS.
Bootable USB Flash Drive – Diskmaker X
Check to make sure your applications are compatible. PowerPC applications are no longer supported after 10.6.
Application Compatibility
Applications Compatibility (2)
El Capitan 10.11 Compatibility information
You will probably needto add RAM. The 2 places I’ve seen recommended most to buy reliable RAM are below. I have purchased RAM several times from Other World Computing and have always been very satisfied with the product and service. They have on-line instructions on how to replace the RAM. OWC has also tested RAM above what Apple states is the maximum. I now have 6GB installed on a early 2008 iMac supposedly limited to 4 GB and noticed an improvement.
Crucial
Other World Computing
