How to connect my iPhone 8 to my Apple Watch

First ensure iphone is updated to the latest iOS - currently iOS16.3.1

Then follow the instructions here to set up your new watch 

Set up your Apple Watch – Apple Support (UK)

It is important that the watch is not currently paired with another iPhone.

You must not have any other sport watch like FitBit paired with your iPhone.

You may need to erase it

Put your watch on its charger and keep it there until you complete these steps. (select the watch OS version) 

Erase Apple Watch – Apple Support (UK)

Erasing all content and settings this way erases all media, data, and settings from your Apple Watch, but it does not remove Activation Lock if it has one. So if it belongs to a previous owner only they can erase it.

Or if it is already under your iCloud account you can erase  and remove it from your account in Find My or https://icloud.com/find .

Wait for the process to finish, then set up your Apple Watch again. Set up your Apple Watch – Apple Support (UK)

When asked, decide to set up as new..