Simply read. Pick up news articles and read them and analyse them, read fiction, read non-fiction, read essays. Ask your teacher for past students' essays and analyse what a good essay looks like. Look at it as formulaic; you need to have a clear introduction. Make sure it flows, make sure it has some leeway to slot in sub-points and spice things up with some nice vocabulary. Although, be mindful of vocabulary and do not bury your reader in a mountain of sumptuous ripostes, jargon and what not. You are here to analyse the text in terms of what it suggests within certain parameters. A good Literature or English essay looks at the literary nuances and fits them into a general theme and what it suggests about that theme under those predications. Discuss the author's potential rationale for inclusion of certain words and their connotations linking back to your prompt (unless, of course, you have set your own "prompt" for Literature). Recognise tone, recognise multiple interpretations.
You cannot simply read a book and do this; you must actually engage with the text. If you just feel words dripping from your pen and expressing exactly how you felt when you read a certain section, you are embedding passion into your piece. Make that nice framework, the formulaic stuff I talked about, have some life. You need to emotionally and cerebrally engage with a text to be at the top of the grading pile. Examiners are responsive to passion in English and Literature, as it makes your piece stand out from the mediocre, run of the mill academic regurgitation from the study guides. By all means use the formulaic stuff in the guides, but do not think that alone makes a great essay.