A cracking essay at A-level has a confident written style and a clear argument that is creatively set up in the introduction; use punchy sentences, attention grabbing adjectives and perhaps an opinion of a critic that you can use as a springboard from which to bounce off your own, new ideas. The central idea on a novel, poem, play etc is then developed in each of your subsequent paragraphs. Within those paragraphs or separately you can include counter-arguments/opinions to your own, but always end a paragraph by reaffirming your own reading/opinion of a text with an appropriate piece of evidence.
Each paragraph in your essay should begin with a key sentence that, if read alone, could act as a pointer to where the essay is going. A key sentence contains a key idea from the central arguement in the essay that will be developed in the paragraph. For example, an essay on Tennyson, the Victorian poet, being fascinated by the Middle Ages might contain a paragraph key sentence like this- 'Tennyson was drawn to the romance, heroism and language of Malory's King Arthur tales.' From this point you would continue to explain why he might have been drawn to the medieval period, drawing on the context of the time, his style of poetry and providing examples from his work to illustrate.
A concluding paragraph should 'hit home' your personal opinion and reiterate why this is the most convincing reading of a particular text.