'A girl was wandering through the forest when she came across an old key...' Continue the story. How do I create a descriptive piece of writing?

One of the most important things to do when writing creatively is to set the scene, because that will engage the reader. Elaborating on the 'forest' mentioned in the question would be a good start, but by including literary techniques such as alliteration or similes in the description will help to keep the attention of the reader. For example, 'a little girl was wandering through the deep, dark forest that was as eerie as an old graveyard at night'. The inclusion of the descriptive language makes the story more interesting. Following on from the initial point of 'setting the scene', it is helpful when writing to prepare a set of bullet points to help frame your story. For example, the introduction of characters, a dilemma that occurs, and how this is overcome, and whether your story will have a happy or sad ending, or will it perhaps end on a cliffhanger? By laying out the journey of your story with a list of things you need to make sure you include in your writing, it will make it easier for you to make the story flow, and you can tick things off your list as you go, making the process as stress-free as possible.