I cannot get the @ key on my Mac. I get the ! instead. How do I fix this?

This sounds like a mis-match between your keyboard and the OS settings - one or other of them may be set for a different language (e.g. you're using a US keyboard with the OS set to a particular European layout, or some such).

You can see from How to identify keyboard localizations - Apple Support that Apple make many different keyboards for international users.

Start by checking System Preferences -> Keyboard -> Input Sources and make sure the selected option matches your keyboard.