How can I watch Apple TV+ on my smart tv?

Without knowing what Smart TV you have that question is impossible to answer. Some Smart TV's have the ability to add the Apple TV app, check with your TV manufacturerer to see if your TV has that ability. If it does, Apple TV+ is within the Apple TV app.

If your TV does not have the ability to have the Apple TV app then you need a device that will allow it, like an Apple TV HD or Apple TV 4K. Please review https://www.apple.com/apple-tv-app/ and you will find a list of devices that support the Apple TV app.