A Single Category, sidebar only showing posts associated with that categeory

I am a novice to wordpress.

I have several categories but I want the sidebar to show only posts that are associated to that category. I do not want to see all posts from all category posts. Just the stories that are associated to that Category for each category page. Like a book. All chapter one showing all chapter to do with that. Then Chapter two with just all posts from that.

I have tried to find an add on and code but it seems to just show all the posts.

Thank you as I am a novice please state where coding would need to be submitted which php file.