You want people to read your essay. For this, you need a strong introduction: announce the point of your essay in an attention-grabbing way and ensure your readers will want to read on. Use language appropriate for your audience and present a summary of your essay in a friendly and interesting way. Basically, a good introduction tells a reader “you really want to read this!”
Most essays will have an argument running through it and will seek to prove or disprove this by the conclusion. Alert the reader to this and whet their appetites for what is to come. No matter the topic, choose your angle. If it is an essay, say, titled “Jonathon Ross: the face of the BBC. Discuss”, the introduction will cover the BBC in brief, Jonathon Ross in brief, and an overview of your argument. Is the BBC good? Is Jonathon Ross good, overpaid, a genius? That’s the beauty of the essay: it is your voice, researched and argued effectively. And the introduction is your launch pad into your well thought through argument, climaxing at the end of the essay with your final conclusion.
An essay needs structure and you should plan your essay prior to writing it. I find mind maps are great for this, with the introduction, middle and end having off-shoots of ideas and points to cover in each section. The introduction normally has the main points to be discussed with a focus on the best attention-grabbing angle, the middle is the argument itself backed up by points of research, and the end is the summary of all that came before.
If you know what you want to say and how to say it, the introduction should be easy to write. But it is always worth playing around with it, tweaking it, keeping it short and punchy. Think “what hook will work with this audience?” If the essay is on the evolution of the earthworm, for example, the piece will need to be factual and sweeping to take in the history and any relevant comparisons; and the introduction may want to say something like “the earthworm began life in the sea and it was only after many thousands of years that the worm of the earth we know nowadays, began to burrow underground” (NB that last bit was made up, I could tell you more about the state of waterfalls in eighteenth century Poland and that is nothing, at all.)
Your introduction is the launch pad to a widely read essay: get it right and people will dive into your work but get it wrong and no one will ever know your mind-bogglingly wise words on the meaning of life, the universe and the earthworm’s evolution.