Write your script first, then create your storyboard.

Script Writing

This is the foundation of your video. It involves three steps, according to AFI's ScreenNation Education program:
  1. Prewriting - establish your goals, brainstorm your ideas, and research your story.
  2. Outline and Treatment - write a brief outline and a summary (treatment) of your idea.
  3. Pitch your idea to your class and get feedback.

Prewriting: AFI's ScreenEducation video on Prewriting.
Outline and Treatment: AFI's ScreenEducation video on Outline and Treatment
Screenwriting: An indepth look at how to develop a detailed screenplay from your script
Making a Good Movie - from AFI's ScreenNation


This video from AFI's Lights, Camera, Education presents a clear definition of what should be included in your storyboard.
Storyboarding from AFI' Screen Education program.

Generate your own Storyboard paper: http://incompetech.com/graphpaper/storyboard/