i recommend the anatomy of story by john truby! its focused on screenwriting but is fairly eclectic, talks you through the steps of generating ideas and making those ideas into solid stories
when it comes to structure, look into things like setup & payoff, the heros journey, and the three act structure - while im a big fan of the first, the latter two arent musts but theyre worth understanding and being able to recognise and put into use, bc they defo work (although using the three act structure without understanding it can be a disaster)
i recommend doing some mind maps of your characters, their relationships and motivations, as well as your world and events that you know take place in the story or want to - build it up from postits so you can move things around before getting a more detailed written-up version of a plot plan