There's no one answer to this question - it depends on what you want your creation story to be about. However, there are a few things you'll need to keep in mind regardless of what your story is about.
First, consider what sort of story you want to tell. Is it a traditional creation story, explaining how the world came to be? Or is it a story about how your particular characters came to be? Either way, you'll need to come up with a detailed and convincing explanation for how everything came to be.
Once you know what sort of story you want to tell, start brainstorming some ideas. What sort of setting do you want your story to take place in? What sort of creatures will populate your world? What sort of conflict will your story revolve around?
Answering these questions will help you start to piece together the basics of your story. From there, you can start filling in the details and fleshing out your characters and plot.
Remember, there's no one right way to write a creation story. So have fun with it, and let your imagination run wild!
