You already know how to write essays, right? You know, start by copying out a definition of one of the words in the question from your Collins Gem dictionary, then fling about some random facts about the author's life (copy and paste from Encarta will do) and bung in some lengthy quotations to fill up space. As long as some phrases from the question are repeated frequently enough, it'll look like you've understood it. Sorted. Essays are such a no-brainer you can safely leave starting them till the night before the deadline...
You've already realized that this is really a description of how not to write an essay at university. The trouble is, it is also a description of all too many essays that your tutors mark. The question you're asking is, how do I stop my next essay being one of them?
Many students' biggest problem is that they do think they already know how to write essays, based on what they did at school or college. In reality, though, there are different expectations about what you write at university level.
So congratulations - by looking at this site at all, you've acknowledged you don't already know how to go about constructing a piece of academic writing. Being aware of, and self-critical about, your own writing process is the first step towards getting better marks for your essays.