my apple pay keeps saying i need two-part Authentication? How do I set this up?

Two factor authentication works like this: you do something (make a payment, try to sign in somewhere, etc). A message is sent to another device (like your iPhone) that has a code.

You use that code to continues with the payment or sign in or whatever.

All that's happening is that two factor authentication is verifying that it's really you doing whatever it is. See, if a crook tries to get into your account, it's unlikely the crook would know to try and steal your phone, too.

You can set up two factor authentication at https://appleid.apple.com