How to update iMac 10.6.8 to 10.7 or higher?

I've been trying to figure this out for quite a while now, and I would appreciate any suggestions on how to update my iMac. I am on version 10.8.6 Snow Leopard, like a few other users here, and I've been trying to update my desktop ever since some applications that I use no longer support this version. I tried updating to El Capitan, as some people have recommended me to do, but when trying to download from the app store, I get greeted with this message: "We could not complete your purchase. This version of OS X 10.11 cannot be installed on this computer." When clicking the "learn more" button, no separate windows are popped up, so I am unable to learn why. I learned later that I am on a much older version of iMac, as when I try to look at "more info" about it, I have an older version, like this one, http://switchtoamac.com/guides/images/about_this_mac_04.png (I am unable to put screenshots into this post). I can't even find the year to determine if my desktop is even qualified to update to El Capitan. I have 2.33 GHz in my Processor, 3GB of Memory, and my Startup Disk is on PRIMUS. I have much more available space than used space, so I don't quite understand why it isn't updating.

I would really appreciate some suggestions to help me with this, and please point out if I overlooked or if you need any more details!

(Please note that I am not the best when it comes to computer software)