To cast your phone screen on your TV you need to have Chromecast.
Follow the steps to successfully cast your screen-
Before you begin casting
1. Connect your phone to a Wi-Fi network.
2. The mobile device, tablet or computer you are using to cast must be on the same Wi-Fi network as your Chromecast.
3. You must be running the latest version of the Chromecast-enabled app. You can always double check that you are on the latest version by visiting the Google Play Store.
Cast from Chromecast-enabled apps to your TV
1. Make sure that the phone you are using to cast is on the same Wi-Fi network as your Chromecast or TV with Chromecast built-in.
Open a Chromecast-enabled app.
2. Tap the Cast button Inactive cast extension. Note: The Cast button isn’t located in the same place on all Chromecast-enabled apps.
3. Tap the device you’d like to cast to.
4. When you’re connected, the Cast button will turn colour, letting you know you’re connected.
5. Select the app like Youtube, Browser, Google Music etc and look for the Cast icon as shown in the image below.
You can now cast videos, movies and TV shows directly to your TV.
To stop casting, tap the Cast button and then Disconnect.
You can also watch this step-by-step guide to do the same
You can check the complete specs of this phone: Xiaomi Redmi 6A