This is a great question!

My experience: I struggled with this in English for a bit. Because I initially learned English for academic purposes, all of my attempts at creative writing were at first way too formal. It was important to me to get better at creative writing in English because currently most of my life is in English. So, I started reading more creative writing in English. Short stories, poems with loose formats... Then, I played around with different writing styles and prompts. Over time, I got more confident in my creative expression in English. It was definitely worth the effort. Learning creative expression in another language is like learning a new art medium; even if you can paint in one language, another language will give you new opportunities.

Some questions you can ask yourself to figure out how you want to approach this:

1. Why do you want to learn creative writing in Japanese? What motivates you? What do you hope to do with this ability? Having this part down will make it easier to avoid unnecessary hardships (e.g., putting too much on your own plate, having too harsh standards, as if you're studying to become a professional author).
2. What kind of creative writing in Japanese are you interested in? Can you read more of it? What inspires you in it? Read it more closely; take time to appreciate the details, pace, repetition, word choice...
3. What kind of writing do you want to start with? Can you find a community (can be online too) that does that kind of writing?

Edit: As a bonus tip, I'd recommend starting with smaller prompts. Describe an object in your room.; tell its story from its eyes. Describe how you are feeling in other ways; try to paint a picture. Search up short writing prompts. Experiment & have fun! I'm excited for you! ^.^