Image source: Cortana
Cortana was a sensation on Windows Phone 8.1. It is not in the All Apps group on desktop running Windows 10 OS. . If you type 'Cortana' in the 'Search the web' on your desktop running Windows 10 OS you get the following:
Click 'Cortana Search and Settings'(system Settings)
Turn OFF to ON.
Cortana welcomes you and before proceeding you need to agree to her terms.
Click I agree and she is ready at your bidding.
Before asking for license agreement she may ask to use the 'Notebook' where user items are stored. If you do not want to use 'Cortana' just turrn it off, but the Notebook items will be saved but not accessed.
These are the items that go into Notebook.
Now your 'Search the web' at the bottom of screen, it will read 'Ask me anything'
If you hit the microphone you get the following and you may need to follow her advice before she can listen to your voice commands.