Well, getting better at fiction writing involves paying attention to details and description. It's also important to study different writing styles and experiment with your own. And always revise and edit your work to make it the best it can be.
To get away with writing good fanfiction, you need to stay true to the characters' personalities and the world they exist in. Also, add your own creative twists to keep it interesting. Don't just copy the original story verbatim.
Just start by picking a favorite character or story you're passionate about. Let your imagination flow and don't worry too much about being perfect at first.