what do I have to do to update the os on an early 2008 iMac?

Obsolete, yes, but still serviceable. One of my family is happily using a 2009 iMac I finished with some years ago.

If you want to upgrade your iMac to 10.11 El Capitan you can download the installer from Apple on this page here. It's a 6GB file.

Scroll down the page to point 4 and you will see the link called Download OS X El Capitan.

After that, plan your hardware upgrade path. Even if you don't wish to get a new iMac a recent refurbished model from a reputable vendor would serve you better for your future computing needs

The latest version of OS X you can install is 10.11.6. I would think about replacing your iMac. It is considered obsolete.