A good introduction is crucial to a good essay, report or any other written assignment. It has two distinct functions:
In other words, your introduction is your map. It should define exactly the aim of your work. Whilst writing, you should constantly refer back to it to remind yourself of the topics you have chosen to discuss, evaluate, analyise or interpret (whichever the aim is) and the order in which topics will be taken.

It is sometimes tempting to turn an introduction into a history of the subject so far. However, an assignment entitled, for example,
'Palaeoecology: Adaption of Fossil Invertebrates to Life in the Sea' should not begin with a long history of palaeoecology. A passing reference to the genesis of palaeoecology, and its area of study, might usefully be made, but it should never be forgotten that the essay is really about the adaption of fossil invertebrates to life in the sea.

When you have written your introduction, ask yourself two questions:

to the aims outlined in the