When first installing macOS from version 10.12 (Sierra) and above, you are given the option to enable Siri.
If you initially declined to activate this feature, you’ll need to first visit System Preferences to activate the service.
Click on the Siri icon in System Preferences and you’ll be presented with several options. (The screenshot below may differ depending on your version of macOS).
You can choose your language, the voice Siri will use and whether to offer audio feedback.
If you’re likely to be using Siri in public places without using headphones it’s advisable to turn this option off.
Then choose a keyboard shortcut used to wake Siri up. You can select from predefined options or opt for Customise.. to create your own.
Finally, you can check the box to Show Siri in menu bar. A small icon will appear at the top right of your screen, clicking on it will activate Siri as an alternative to using a keyboard shortcut.