I lost my phone but all my contacts are on my Apple Watch. How can I move them to my phone?

If you have been making regular backups of your old iPhone, you should be able to recover the contacts when you setup your new iPhone. See step 5 in:

Set up your iPhone or iPad - Apple Support

You should also put your old iPhone in Lost Mode:

If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support