If you are looking to just play a video to a wider audience rather than mirror your entire iPhone screen, you can make use of the AirPlay function built into many applications that are available on the app store, including YouTube.
Step 1: Locate a video that you would like to stream to a TV using an app such as YouTube.
Step 2: Tap the video that has started playing and select the streaming icon (as shown below) and choose the device you would like to connect to.
Step 3: If you are playing a video from the photos and videos that are stored on your Apple device, tap the ‘Share’ icon and select the ‘AirPlay’ icon from the list of options. You will be asked to choose a device to stream this content to.