To start making a good RPG story, you should first consider what kind of experience you want players to have. Do you want them to feel heroic, or maybe more like an underdog? Based on that, you can start creating the setting. It could be a post - apocalyptic world, a high - fantasy realm, or a steampunk city. Once you have the setting, start thinking about the events that will drive the story forward.
Start with a simple premise. For example, a young hero who has to save their village from a dark force. Then, expand on it. Develop the characters that will be involved in this story, their relationships, and their motives. Also, do some research on different RPG story tropes and see how you can either use them in a new way or avoid them altogether.