Ii can't power off my apple watch- when I press the side button I get siri.

It sounds like you're pressing the Digital Crown, not the side button. Make sure you're holding down the side button, not just pressing it once. See the article below for clarification:

How to restart your Apple Watch - Apple Support

I assume you had it paired with an old iPhone prior to getting the new one? If so, you'll need to break the connection with that old phone first before you can pair it with the new one - see How to pair your Apple Watch with a new iPhone - Apple Support.

Once you've paired the Watch with the new iPhone, you can either Change the watch face on your Apple Watch directly or use the phone's Watch app to Explore the Face Gallery on Apple Watch - Apple Support.

Explore the Face Gallery on Apple Watch - Apple Support