How do you analyse a novel/short story?

First of all, know the story. Study guides or even talking about it with a friend or teacher can help you learn and understand the story and the themes that run through it. Once you've got this down, the rest is easy! 

Your interpretation of the language of the novel/story is the most important part of your essay. This is how the author put across their point, and why. This 'how' part is looking at the way in which the author portrays emotions and thoughts. The author will use literary devices such as metaphors, similies, speech and irony in order to demonstrate a certain meaning or feeling. In the essay one should focus on the effect the language has. This should always be supported by quotes from the text, and any quotes should be thouroughly analysed. 

An example discussing "To Kill a Mockingbird" by Harper Lee; The author uses similies to allow the reader to better visualise the setting. For example when Lee describes the ladies of Maycomb as "soft teacakes with frostings of sweat and sweet talcum" it allows the reader to visualise the sickly sweet setting and perhaps suggests that their more unpleasantly human qualities and behaviours (such as sweating, but also their thoughts) are only just hidden under a thin layer of sweetness. 

This is just an example and you dont need to know the story to understand how the interpretation can work. It can be applied to any novel and can go into as much or as little detail as you want. The best essays interpret just a few points but go very deeply into each. It is quality, not quantity of points you are looking for.