When you start thinking about an essay, you need to orientate yourself in relation to the question by finding a way of approaching it. Any question set for a university essay probably allows several different ways of approaching it, which are all equally good. You'll have to narrow down the possibilities by thinking of things you've come across that you might use as examples.
The first step towards making a plan is to think about the rhetorical situation of your essay. What is the question really asking you to do? What expectations is your essay supposed to fulfill? What kinds of assumptions are you going to make about your audience? It can often be a good idea to disagree with the terms in which the question is put, provided you can back this up with argument.
The next step is actually to draw up a basic plan. It is useful to do this at an early stage in the writing process, as it helps you to form a mental picture of the ground your essay needs to cover. Using mindmapping is a good idea at this point.