You can use your phone, tablet, or computer to control the YouTube experience on TV, using the Cast icon to Cast or Airplay. If you’re casting from a mobile device, make sure to download the YouTube app. Casting from youtube.com in a mobile web browser is not currently supported.
Set up your chromecast: https://support.google.com/chromecast/answer/2998456
Learn more about casting to your TV: https://support.google.com/youtube/answer/7640706