Adding data fields to watch app

Have programmed my first Watch Faces and now want to take the next step towards Watch App. I have already looked at the samples, but unfortunately I have not yet really figured out how to start here.
My idea is to program a new activity. First, I would like to create the possibility that the user can add the various data fields as with any other activity. After that I would like to add more screens where different other things are displayed. Unfortunately, the first step is already difficult for me. How can I create a "standard activity" to add the data fields?
Is the code of any standard app (like running for example) open source?