What to do if my old Apple Watch won't connect to new iPhone?

I have a new iPhone and i have an old watch. Its my watch its connected yo my apple id and i know my passcode. However i no longer have the old iPhone that my watch was connected to originally. I know all my passwords and codes so why is it so hard to swap my watch over? How do i do this? Please help?

[Re-Titled by Moderator]

Original Title: Connect watch to new iphone