How to Sign in to Apple TV?

For the Apple TV app on third party devices, find the in-app Settings (⚙️), then Accounts, and enter your Apple ID account credentials there. Sign in to the Apple TV app - Apple TV app user guide - Apple Support

For the Apple TV box (4th generation or later), go to Settings﹥Users and Accounts﹥ and sign in there. For previously owned devices, it is highly recommended to first reset to factory settings.

For other devices, please describe what you have.