If you have an Apple TV 4th Generation (newest version), you can download the Tubi TV app directly from the app store, just search for "Tubi TV", install it, and you'll be all set!

If you have a different version of the Apple TV, you can use Airplay to stream from your iPhone, iPad, or iPod touch to your TV. You can find more information here: How to Airplay to my Apple TV.