The Apple Watch Series 3 and 5 that Apple is currently selling come with watchOS 6 that require an iPhone 6s, SE, 7, 8, X, XS, XR, 11 or 11 Pro running iOS 13
The iPhone 6 will not pair with your iPhone.
If and only if you get a old Series, not new, or from some store's old inventory, then it might work, but you have to be very careful that the previous owner did not leave the Apple Watch registered with their Apple ID.
What to do before you sell, give away, or trade in your Apple Watch, or buy one from someone else
https://support.apple.com/en-us/HT208855