I have an old Desktop Mac and am unable to connect to any website. I have confirmed I have connected to the internet. Safari cant open the page cant establish a secure connection to the server

Great advice so far...

Mostly you just need a more modern Browser than Safari on Catalina, Mojave, or High Sierra like Firefox or Brave.

https://brave.com/

https://www.mozilla.org/en-US/firefox/83.0/system-requirements/

Details about recent download Brave, for 10.13.6/10.14.6 are here:

<https://github.com/brave/brave-browser/releases/tag/v1.57.64>

Firefox 83.0 System Requirements (mozilla.org)

Opera…Thanks

https://www.opera.com/download/requirements

Opera system requirements | Opera

Thanks to dialabrain…

Firefox 115 is now the current version. runs in High Sierra.

119.0.1 is now the current version and requires 10.15 or later.

https://www.mozilla.org/en-US/firefox/new/

Thanks to A Brody, Browsers by Mac operating system…

Browsers by Mac operating system - Apple Community

if you have a 2015 iMac, and are still using the macOS that was installed at time of purchase (maybe El Capitan?) then that would explain those errors from most if not all websites. the safari version for that OS is VERY out of date. you could try and upgrade to a newer macOS. when you do that, safari will also be updated.

please use Apple Menu > About This Mac and let us know exactly which model and model year of Mac you have. and also the current macOS you have. from there, we can tell you if you can upgrade, and to which macOS.

You simply need to upgrade macOS. To learn how to do that please read How to download and install macOS - Apple Support.

The Safari version incorporated with OS X 10.8.5 will not work with most web pages.