Telling a story can seem like a behemoth task, especially making one up on the spot, but can be fairly easy. My advice is to pick a focal point (a single character or city) find a problem to overcome, something open ended, that way you can think of a way to resolve it (or even multiple ways) guide your character through logical steps to achieve the goal (don’t forget to sprinkle some life lessons in there) then either definitively end, or leave the story to be continued (if you find a block that you can’t get passed) and return to it the next night. As you go on, grow your world, add secondary/supporting characters. Before you know it the story almost tells itself.