Well, you could start from the very beginning. Think about the most important event or moment that sets the whole story in motion. For example, if it's a story about a trip, you might start with the decision to go on that trip.
Well, in a fanfic with Kirishima and Bakugou, it could be about their friendship. Maybe they are facing a difficult challenge in their hero training and they have to rely on each other's strengths. For example, Kirishima's tough physical form and Bakugou's powerful explosions could be combined in a strategic way to overcome an enemy. Or it could be about how they grow as individuals while also strengthening their bond as comrades.
It could mean that I'm going to be something that haunts you, like a memorable or spooky experience that stays with you. Maybe it's a metaphor for leaving a lasting and somewhat eerie impression on you.