What Makes a Good Novella?

What Makes a Good Novella?
In order to understand what makes a good novella, you'll have to read some. That's a pleasure, and you'll also learn what you need to know.
Of Mice and Men by John Steinbeck or The Metamorphosis by Franz Kafka are among the most famous of novellas. They both have quite recognisable themes.
So firstly, in order to get that succesful novella on the go, you will have to choose a central theme or idea. The most successful novellas tend to focus on a singular powerful theme; such as loss, regret, love etc. As a novella is a piece of short fiction, it is ideal for exploring a specific topic at length. 
Secondly, you must create a captivating character. As with all short fiction your character is important, because your character is the focus of the story and the medium for your reader, and they must be able to engage with them on an intimate level. 
Thirdly, you must choose a dynamic setting. Choose a setting you know a lot about or have a lot to write about. Settings can act as another character in your story, so you must give them the appropriate amount of thought and consideration. 
After you are happy with the outline of your novella you can move on to creating your first draft. Some further advice would be to create a strong opening, focus on a singular conflict, keep subplots and secondary characters to a minimum, concentrate the story around a specific time frame and finally close the story with a powerful ending. 
Good luck, and have fun with it.