How do I get a certificate to install Mojave?

I'm not sure what you mean by "a certificate to install" but macOS updates are available from the Mac App Store or Apple Menu > About this Mac > Software Update.

Regarding the mid-2012 Mac Pro, you must validate that it has the required configuration (Graphic card).

How to upgrade to macOS Mojave - Apple Support

Hope that helps,

WD

ThunderBus1104 wrote:

How do I get a certificate to update to Mojave for a Mid 2012 Mac Pro?

Mac Pro: Late 2013 or newer; Mid 2010 or Mid 2012 if upgraded with a recommended Metal-capable GPU

https://itunes.apple.com/us/app/macos-mojave/id1398502828?mt=12