As someone who primarily makes narrative-driven games, I always start with the story; after all, narrative games are held together by their stories. If you struggle to develop ideas and characters, consume media (games, books, movies, and TV shows) or brainstorm with other writers and game developers.
When it comes to writing, you can either outline first or dive straight into it. I prefer to outline first to build a foundation I can build on, but use whatever approach works for you. There are lots of resources that can help you with your writing journey:
- Abbie Emmons (novel-writing and character advice; you can still apply this to narrative games)
- The Closer Look (writing advice and media analysis)
- Arimiadev.com (visual novel resources and advice)
- Visual Novel Design (visual novel development, writing, and advice)
Ultimately, my biggest advice is to play and study other narrative games. Note down how the mechanics work, how the story is structured, what you like/dislike, what you think could have been done better, etc. Game jams are the best places to find amazing free games! In addition, read books and watch movies and TV shows, not only to broaden your horizons but also to study how stories are told across different media. Experimenting is always fun!
I hope this helps! If you have more questions, let me know ^^ Good luck with your game dev journey :D