How do I download the YouTube tv app to my Apple TV?

You didn’t mention what generation of appleTV you have and it really matters.

Only older generations of Apple TV allow downloaded apps from their App Store, that aren’t pre-installed. Hopefully that’s built into yours but if you have no App Store on there, then you prolly have a third generation & are out of luck… but you do have an option below:

”If you can't find the App Store

The App Store isn’t available on Apple TV (3rd generation). If you have this model, you can't download new apps, but you can update your software to update your built-in apps.”