Your introduction should tell the reader what to expect from your essay. Stay focussed on the question, and keep it brief.
Do not give very broad background information on the general topic, but focus instead on what is relevant to answering the set question.
What should be in an introduction?
There isn't one way to write an introduction, and following one particular structure could lead to your introductions becoming very formulaic.
You might introduce the main subject of the essay and why it is an important topic. You may also provide definitions for any ambiguous terms or concepts. Your reader needs to know what you mean when you say certain things. This is usually only necessary when there are terms that have numerous definitions: eg if you use "consumerism" are you discussing this as an ideology, economic policy or type of behaviour?
Your introduction may also provide an outline of the key argument(s) presented in the essay and how you are planning to answer the question.
How long should an introduction be?
An introduction usually makes up 5–10% of your whole essay, although there is no absolute rule.
The amount of detail that you can include in your introduction will depend on your word count.
This is an example of a concise introduction:
“Concern about racism in the police has increased since the 1980s. A number of high-profile cases have highlighted that ethnic minorities have not received treatment equivalent to their white counterparts. The focus of this concern has been whether racism operates at the individual level or whether it is embedded in the policies and practices of the police (Easton and Piper, 2005). Two significant inquiries, which were carried out in this period – the Scarman report and the Macpherson report – will provide a focus by which to critically analyse the concept of institutional racism in policing and evaluate policy responses. Whist some progress has been made since these two inquiries were carried out, many improvements still need to be made to overcome institutional racism.”
Take a look at this detailed example of an introduction (PDF), which is broken down to show the purpose of each sentence within the introductory paragraph.
Both versions are well executed but written in different styles and for essays with different word limits.