Writing prompts are designed to kick-start your creativity. A writing prompt is simply a topic around which you start jotting down ideas. The prompt could be a single word, a short phrase, a complete paragraph or even a picture, with the idea being to give you something to focus upon as you write. You may stick very closely to the original prompt or you may wander off. Read the prompt and see where it takes you.
Think about bananas, the color, texture, smell.
Write five memories about bananas that come to mind.
Now use those memories in a story. Start with: He was my childhood best friend...
