When you are done writing your story, from inside the editor: tap the 'Save and Exit' button in the left hand corner (for Poems, choose 'Menu' › 'Publish').
Fill in as many details as you like on the following page. For picture books and longform formats, you may choose to remain in Draft mode, in which case your story will not be published. Otherwise, choose to publish your story Privately or Publicly (Students do not have the Public option). Poetry does not have a Draft option.
Tap "Publish" at the bottom of the form when you've added your details. Done!
Note: You can change any of these at any time as often as you'd like.