If you think of the introduction being 10% of the word count (and your conclusion being slightly less than this) your essay, whatever type and whatever length, will be well balanced.
The introduction, which opens the discussion of the topic, should always include the following:
- your thesis statement
- some background information (which might include reference to previous research)
- why the subject is important in the field, which is really your rationale for choosing the topic
- an idea of the essay’s organisation in the order in which you will write it
A longer type of assignment such as a dissertation might also include important definitions in the introduction. The introduction to a 10,000 word dissertation will probably be the first chapter of the whole paper, but it will still be 10% of the word count.
Some people suggest making notes for the introduction and completing it when you have finished writing everything else. This way, any changes to your thesis and the essay organisation will not need too much rewriting. Some students find it difficult to approach essay writing this way and prefer to just write the introduction and make changes later if necessary. Whichever way suits you best, you should have an outline mapped out before you start on the text of your essay.