Setup · once
Turn Dictation on
Open System Settings, click Keyboard in the sidebar, and scroll to the Dictation section. Flip the toggle. macOS asks you to confirm, and may download a language file.
On macOS Monterey and earlier, the same switch lives in System Preferences > Keyboard, under the Dictation tab.
While you're there, check the language and which microphone it listens to. "Automatic" picks the built-in mic, which is usually right unless you use a headset.