By now you should know who you're going to work with, and what you think makes a good story. Before you start, it's smart to make an overview of what you want to include in your story. Sit together with your partner(s) and answer the following questions.
Characters
who are the characters?
what is their past/present/future?
who lives/dies?
what are the relationships among them?
Setting
where/when does it take place?
what effect does this have on the story?
what locations/times are we in?
Theme
what is a general message you’re trying to get across?
what does the audience need to take with them?
Conflict
what is a turning point?
what events lead up to the conflict?
Besides the basic elements of a story, you should also include the following in your preparation:
Storylines
explain how different storylines are intertwined or paralleled, how do they influence each other.
Plot devices
Look up: MacGuffin, Deus ex machina, Shoulder angel, Red herring, Plot voucher. Include one of these in your story and explain how it is included/why it works in your story.
Materials
Make a list of the materials you will need to actually produce your story so you can bring them next time.