how can I watch what is on my computer on my tv using apple tv?

A couple if different ways depending on the content, and where its stored, and whether you have a Mac or PC.

Purchases from the iTunes Store, can be accessed directly in the Tv App and Music App if both the computer and the Apple TV are using the same Apple ID: Watch iTunes movies and TV shows on Apple TV - Apple Support

Content in the iTunes application on a PC or Mac or the Music or Tv Apps on Catalina on a Mac that was not purchased from the iTunes store but imported from other sources, can be accessed through the Computers app on the Apple TV using Home Sharing: Use Home Sharing to share media from a computer to other devices - Apple Support

Outside of that, If your computer is a Mac, you can Airplay from your Mac to your Apple TV, and mirror your Macs' screen onto the connected TV screen: How to AirPlay video and mirror your device's screen - Apple Support

Finally, you can connect your computer directly to your TV with an HDMI cable, and use the TV as its own screen.

If you would like to watch your Apple TV Plus content:

Windows PC or Microsoft Surface:

• To access your movie and TV show purchases, use the latest version of iTunes for Windows
• To sign up and use Apple TV+, go to tv.apple.com

On the web:

• Go to tv.apple.com

if you have a Mac running MacOS Catalina, there is the Apple TV App to use, otherwise use the above web link and sign in.