How can I write a good introduction

An introduction begins an essay in the most clear and concise way possible. It's important to immediately state what texts you'll be analysing and refer to any authors using their whole name the first time. After that you can introduce the purpose of the essay, what you are defining or examining, and what ways you're going to achieve that. An introduction is also the best place to define or redefine terms (within the framework of your argument) as you're going to use them throughout the essay, for the sake of clarity and specificity.
For an exam you can even have a few stock sentences on any given text to begin. It is important not to give the plot-line of the text as this will eat up time/word-count and just put off getting into the heart of the argument. It is nice in an introduction to use enticing language to engage the reader/marker from the beginning and show your passion for the subject. Ultimately an introduction is just to nicely set up the main body of your essay and that's where you'll gain nearly all of your marks. Keep it brief and as clear as possible, then dive into the argument.