Cortana’s finally made the leap to the PC in Windows 10, assuming many of the operating system’s search functions, even though her first iteration could still use some polish. (That’s what Previews are for!) But by default, she doesn’t listen for your commands.
If you’d like to be able to just bark commands at your PC, open Cortana by clicking the search field in the taskbar and select the three-lined options menu in the upper-left corner. Select Settings for the list, then simply enable the “Let Cortana respond when you say ‘Hey Cortana’” option. You’ll need an active microphone for this to work, of course.
While you’re poking around Cortana’s options, you can dive into the Notebook menu to fine-tune exactly what personal data Microsoft’s digital assistant can access. Remember, however, that like Google Now, Cortana’s effectiveness is directly related to how much she knows about you.