Exercise. Anything to take your mind off of your story or poem or whatever. But no video games, those aren't very good for the creative process.
Draw a picture. If you simply have no ideas to write a new story, just start making a weird and wacky scene, while coming up with a story in the picture.
Write. If you can't write a story, write abiuot your day, or something new you noticed while walking home from school. Even if you aren't writing a story, you can still use these details another time.
Describe something. Even if the right words don't flow to you, describe the colors of an apple, the shape of a pear, the smell of a pencil. After doing this, you may right a poem about one or the other.