How can I effectively plan an essay (on chosen book)?

The first step to writing an essay is fully understanding what the question is asking of you. Take, for example, a question on 'Of Mice and Men'.
What is the importance of dreams in Of Mice and Men? What role do they play in people’s lives?
Pick out key words from the question. Here they would be "importance", "dreams" and "role". The question isn't looking for you to simply highlight the main characters who have 'dreams', but to understand their significance in relation to the novel as a whole. Choose the characters who would be useful to answer this question. A full detailed analysis of 2/3 characters is far better than a brief analysis of ALL the characters who have 'dreams'. Next, plan your essay paragraphs briefly using the P.E.E. system (Point, Example, Evaluation). After this, you can work on the Intro as you now know the format of the essay bulk.