A Watch app complements your iOS app; it does not replace it. If you measure interactions with your iOS app in minutes, you can expect interactions with your Watch app to be measured in seconds. So interactions need to be brief and interfaces need to be simple.
Read up on the just released guidelines on how to create a well-designed Watch user interfaces. There's also a video describing the development process, and how iOS interacts with WatchKit, on the main Apple Watch Developer page.