Start with a stimulus, go google some pictures of landscapes or something that might inspire you to write.
or... just force yourself to use this stimulus/question:
'I wanted to keep on moving, yet I wasn't sure where too"
Using the statement above as the beginning to your story, compose a piece of imaginative writing which expands on the notions of journey.
... Other than that my best advise is to just start writing something, it's all about being creative so don't be shy when it comes to writing about something you usually wouldn't. You'll find that what you write might not be the best but if you aim for a solid 800-900 words your going to have something that resembles a story, then once you have this you've got yourself your first creative writing story.
Edit this story that you've written to make it better and more cohesive or simply just learn from your mistakes by finding out what made this story not so good and try again with a new stimulus.