Think about a fantasy story that intrigued you. The images that come to mind will encompass the elements of the world itself (the setting), the characters and their quests (heroes and antagonists) and the magical beings that either help or hinder the success of the quest.
While not all fantasy includes faeries and other such creatures, many do, especially those for younger readers. Belief in alternate realities inspires many children to explore different types of stories.
If you intend to write fantasy, one factor that you need to consider is how to populate your world. Will there only be humans in conflict with other humans as they attempt to find or steal some type of object (such as the Holy Grail)? Or will orcs, wargs, ogres, wizards, trolls and other magical beings participate in the telling? If so, how will they enrich the story?
Your task is to write a story in which at least one type of being plays a major role. First do some research to discover the known options. If none of them appeal to you, create your own by beginning with its physical appearance and then by bestowing a combination of uses and powers.
How will the presence of this being influence the story? Will it be friend or foe to your protagonist? In what ways will it affect the telling? It can either move the action forward or slow it down by presenting obstacles for your character.
Have fun with this one.