How do I write a good introduction?

The key to this is remembering that introductions rarely score additional marks on exams, and so a short but sweet introduction is perfectly fine! 

The most important thing is to avoid phrases like "in this essay I will/am going to", as these really don't set a good tone for your essay! Try starting with a quotation, to grab the attention of your examiner, or a succinct definition of the main theme of the question, for instance:

Discuss the use of violence in Macbeth

Using method one: "Bleed, bleed, poor country!" Macbeth's Scotland is a nation steeped in gore and violence...

or How far do you agree with the idea that Lady Macbeth is a transgressive female?

Using method two: Transgression:an act that goes against the law, rule, or code of conduct of a society. Whilst we can argue that Lady Macbeth... (also there's a good opportunity for a nice rule of three in method two!)