The introduction sets the scene for the whole essay. For a 1500 word essay, this could be a 150 word paragraph. The purpose of the introduction is to identify the topic, your point of view and the main reasons for your response.
Often this paragraph includes:
An introductory sentence or two related directly to the question.
A definition or explanation of a key term, cited from your discipline.
A clear statement of the focus of the essay or the position being argued, your thesis statement.
Sentences that give a clear idea of what the essay will be about (content) and the direction (scope) the essay will take.