How do I write an introduction for my coursework essay?

Your introduction should briefly set up the parameters of your essay; and touch on the themes and topics that you're going to cover in your essay. It's essentially the tl;dr of your essay, but you have to write it in such a way that the reader wants to read the rest of your essay. It's not a summary of the plot of the text you're writing about.

It's also important to provide context for the text; when it was written, what role it played in the author's development as a writer, whether the author's previous work inspired the text or whether the text inspired the author's later work. It's also useful to set up the historical context of the piece; if you're writing about a novel written in the post-war period, or if the writer moved in particular circles that had an impact on their work (e.g. Virginia Woolf and the Bloomsbury set).