I have ordered apple tv plus on my firestick however, when I want to watch a movie I get "How to watch....what am I doing wrong?

• Make sure to be signed in to the Apple TV app with your Apple ID account.
• Unfortunately, you can’t subscribe or make purchases for Apple services on Fire TV devices [1]. There is currently no agreement between Amazon and Apple for in-app purchases. You will have to do the purchasing/​renting/​subscribing part on a different device. When that is done, you can view the content, that is then linked to your account, on the Fire TV device.

I hope you realize that Apple TV+ consists solely of Apple’s original content, nothing else is free. You if you pick a movie anywhere else in the Apple TV app you will get this message and the option to rent or purchase the movie.