[edit] Steps

Look around you. Do you see anything with a story behind it? Like that necklace grandma gave you or the blue ribbon for the soccer tournament. Put that story down on paper!
Look online for a list or writing topics. You can find plenty of topics that you would have never thought of!
Think about movies and books you've enjoyed and what made you enjoy them. Was it a certain type of character or a setting? Try incorporating similar elements into your own story.
Watch random people walk down the street and write a pretend life story for them.
Think of an event that has happened in your life recently, or maybe something that happened a long time ago.
Write ABOUT your favorite book or movie.
Write about the dream you had last night.

[edit] Warnings

Do not base your story off another book or movie's ideas. This is called plagiarism and it IS illegal. Even if you're not caught, someone will notice.