Think of your favourite character and/or world from fiction. It can be from a book, graphic novel, tv show or movie.
Now write a short piece using that character and/or world.
I don't mean fanfic. No stories about the firefly class spaceship Serenity stopping by your house to pick you up for adventures with Captain Malcolm Reynolds (or is that only my fanfic story?).
Anyway. Write the piece portraying the character and/or world as accurately to the source material as possible. This will make you appreciate how well written the original character is and the tricks the author used to make the character fleshed out, 3 dimensional and likable.
And remember - this story is for your personal writing file. I don't want to encourage anyone to be a plagiarist. :)