Well, starting an Ajax fanfic requires some preparation. You should immerse yourself in the world where Ajax exists. Read any official materials related to him, watch any relevant shows or play the games if he's from a game. Once you have a good understanding of Ajax, brainstorm ideas. Consider what kind of story would be interesting and unique. Maybe it's a story where Ajax teams up with an unlikely ally. Then, plan out the structure of your story, including the beginning, middle and end. You can start with a strong hook to draw readers in, like a mysterious event that involves Ajax right at the start. And don't be afraid to let your creativity run wild as you write.
First, get to know Ajax well. Study his character traits, his backstory if there is any. Then, come up with an interesting idea for the story, like a new adventure for him. Start with a simple plot outline and build from there.