Many of us probably think that writing works this way: You "have an idea" and then you "write it down." I want you to question that assumption. In an important sense, you don't really know what your ideas are until you've written them down and phrased them just right. Writing can be a process of discovering what it is you have to say, not just of communicating a prefabricated idea that's sitting up there in your brain. As long as it's up there in your brain, the idea doesn't really exist in any form communicable to another. It exists, at best, in potential, but it hasn't been subjected to the rigorous test of enunciating it. So use your writing to figure out what you want to say and how you should link each thought to the next. This may mean that, rather than making a complete outline of your argument and then filling it in with your prose, you simply start writing and see where you go. As you proceed, you may find (you're likely to find) that you actually want to move in directions different from the one you originally imagined yourself taking. That's fine. But when you're done, go back to the beginning and make sure the introduction of the essay fits the directions you actually wound up taking.
I have little doubt that if I were to ask you merely to summarize the plot of a work we're reading in this class, most if not all of you could do that in perfectly acceptable clear prose. I find, however, that when I add the significantly greater conceptual burden of requiring students to write argumentative essays that defend particular theses, the quality of many students' prose declines markedly. Let's appreciate the correlation: If a student is confused about her thesis or the proper way to argue for it, she is likely to write worse than she would if she were simply summarizing. A student arguing from a clear and cogent thesis, on the other hand, is much more likely to write well. Developing a worthy thesis statement is therefore crucial: You should try to present a one-sentence statement of the argument you will be making. (Sometimes the statement of your thesis may run to more than a single sentence.) Often this appears at the end of the essay's first paragraph. The statement needs to claim something that it is possible to doubt (otherwise it would be so obvious that one wouldn't need to argue for it) but that can be made plausible by careful marshaling of evidence. In this class, your evidence comes from the text(s) you will be writing about, so be prepared to quote a good deal. In keeping with point 1 immediately above, you may find (you probably will find) that you are not ready to articulate your thesis statement right away, but that you have to write a bit in order to discover what it is you want to argue. So give yourself time to discover your thesis, then put it where it belongs near the start of your essay.
Don't assume that you can't use the first-person voice. "I think" or "it seems to me" is Ok, as long as you have reasons to back up what you think. I'd rather have you sound like a distinct individual than an impersonal bureaucratic committee. This advice also applies to the passive voice: Make active voice constructions your default ones, and reserve passive for the rare times it really comes in handy.
Begin your work process by saying to yourself, "In order to assemble a persuasive argument about this topic, the passages in the work I'm writing about that would really need to be looked at are …." In other words, make a list of all the potentially useful passages in the work that address the question you're trying to answer. Then prioritize your list to focus on those that seem most important, i.e., least permissible to omit from a short essay. This exercise provides training in qualitative assessment of information. It can also help safeguard you against the criticism that you have omitted passages that might seem to contradict your argument.
Quote from and / or paraphrase portions of the text to support your claims as much as possible. Don't think of your task as one of arriving at the tersest statement of a book's "meaning"; think of it as one of engaging with some issue raised by the book in sufficiently satisfying detail. Occam's razor doesn't apply here. Back up and / or illustrate your assertions by showing how they are borne out by the work you're discussing. More richly detailed essays, those that take the trouble to respond very fully to the text and to the questions posed in the assignment, will receive higher marks than those that give minimal or very general responses. And it never hurts to write more than the bare minimum number of words required, so long as all your words count toward enriching your argument and none of them is just there as filler.
In high school, many of you may have been taught the model of the "5-paragraph essay" (opening paragraph, 3 paragraphs of "body," closing paragraph). That was fine then. College-level work takes on more complex subjects or looks more deeply into subjects you may have encountered earlier. This means that the 5-paragraph model, rather than helpfully providing structure, can become a cage, involving a boredom-inducing level of repetition (here's what I will say, here I am saying it, here's what I have said). You want to persuade your readers, not hit them over the head. Acknowledge the greater sophistication of your material, your approach, and your imagined audience by bidding the 5-paragraph model a grateful farewell.
Don't write for the professor. Write for an imaginary reader who is somewhat familiar with the text you're discussing but needs some reminding about what happens when and so forth. When you discuss a particular passage or incident in the text, make sure to contextualize it for this imaginary reader. When you move from one part of the text to another, help the reader move with you by providing a sufficient transition. Always be asking yourself if what you're saying will be clear and persuasive to the imaginary reader.
Keep your opening paragraph focused on the specific claim your paper is making. Don't begin with vast generalizations (e.g. "Throughout the history of Western literature") when your real subject is something specific about a particular text.
People often think that academic writing is unlike the kind of writing they might need to do in the "real world." But one way in which the writing in this class is exactly like the writing you might do in a professional context is that it is being read by someone busy who has a lot of other things to read and who will hold you accountable for what you present.