First choose an interesting character as a protagonist and another as the antagonist - and perhaps another to add interest -that provides the who. Since this is a fairy tale, it would do you well to describe your characters in detail giving physical traits as well a character traits.
Choose what happened to these characters- Put them in a romantic or magical or dangerous situation and describe the events.
Choose where and when. Perhaps the events take place in a castle or a forest - 2 common fairytale sites during the time when knights, and princesses and dragons roamed the earth.
Tell why and how it happened. What did the protagonist or antagonist or other characters do to put them in the place and the situation they're in.
Since this is a fairy tale it would do you well to describe your characters in detail giving physical as well character descriptions.
Use colorful language and figures of speech to have the reader form a mental picture of your setting and characters. Use lots of adjectives and adverbs, similes and metaphors as well as color words.