The Member Center of Apple's Developer website
Xcode's project settings
iTunes Connect, Apple's portal for app administration
As stated already, this section is not intended to be a catch-all guide to App Store submission, but rather aims to provide the context in which additional watch-related steps must be taken.
We are assuming here that you have registered as a developer, that your Development Program membership is valid and up to date, and that you have currently valid development and distribution certificates. Please refer to Apple's documentation for further information.
Log into the Member Center at https://developer.apple.com/membercenter/index.action and click on the Certificates, Identifiers & Profiles link.