@ and " keys are in opposite place on my iMac keyboard

What language is set for the keyboard input in Settings ➜ Keyboard ➜ Text Input / Input Sources ➜ [Edit]

On a US keyboard layout for instance, @ is obtained by pressing shift+2 keys together, while " is obtained by pressing the shift+' (apostrophe) keys together.

Where exactly are you expecting there @ and the " to be for you?

Is it an English layout? Or some other language?

jackyfromworcesterpark wrote:

On my brand new iMac and keyboard, When I press " i get @ and same in reverse

That usually means you have the wrong input source active -- probably British PC instead of British or vice versa.

Go to system settings / keyboard / text input / edit / all input sources and use the minus and plus buttons at the bottom to make sure you have just one on the list, and that it is the right one. For @ on shift 2 you want British, for " on shift 2, you want British PC.