As you are writing your first draft, "try to write as much as possible at one sitting to ensure continuity in your train of thought....As you write each section (of the essay), be sure it is clearly related to the main idea and to preceding and following sections. Transition from one section to another is more difficult in research papers than in short papers because the greater length of a research paper makes it harder to keep all the parts in mind simultaneously. You will probably find that you need to write some transitional sentences or paragraphs" (Millward and Flick, Handbook for Writers 395-396).
The first paragraph introduces the topic and sets forth the thesis.
The middle paragraphs explain and illustrate your thesis. They set forth the reasons for your thesis in the order of increasing interest and importance, and, in the case of an argumentative essay, confront and dispose of the opposing point-of-view.
The end paragraph is a summation and reassertion of your thesis. It ends on a strong note: a clincher.
In the first paragraph, introduce your topic and prepare the ground for your thesis statement that comes at the end of the first paragraph. The first sentence of your first paragraph should capture the reader's attention and point the reader in the direction of the thesis statement to come later. Here is an example of a beginning paragraph. It is from an essay entitled "Long Live the Wolf!" which criticizes the provincial government's ongoing program of shooting wolves in northern British Columbia:
When man rests his considerable weight on the delicate scales of the balance of nature, the results can be disastrous. Nature's harmony, the intricate plant and animal relationships arrived at after tens of thousands of years of co-existence, can be dashed in a relative instant by the intrusion of man.
Of course the stated motives for this interference are nearly always altruistic, but it usually doesn't take much scratching of the philanthropic veneer to uncover the real reason - human greed. The provincial government's exercise in conservation, known as the wolf-kill program, is an example of human interference with nature for the sake of greed. By shooting scores of wolves because it claims the wolf is a menace to the north, the government is actually endangering the existence of the wolf species and destroying the natural ecological balance - - in order to line the pockets of some of its already wealthy partisans.
Notice how the whole introduction funnels into the thesis statement.
The middle paragraphs will elaborate your thesis. Each middle paragraph will take up one main idea in support of your thesis, explain it, and give examples of it. To a good extent, the strength of your essay will rest on the power and relevance of the evidence presented in the middle paragraphs.
Your ideas should be arranged in order of increasing interest and importance, saving your best idea till last, since you want your essay to increase in intensity.
While defending your own ideas, you should acknowledge and dispose of the opposing point-of-view. You may devote the first or second paragraph of the essay to the opposing viewpoint, and then spend the remainder of the essay picking apart those ideas by explaining your own viewpoint. Or begin each middle paragraph with one aspect of the opposing viewpoint which you then counter with your own idea. Here is a sample middle paragraph from the essay on the wolf-kill program:
The supporters of the wolf-kill program claim that the wolf has decimated the elk and moose herds. True, the wolf does attack elk and moose, but its prey consists of the older, the sick, and occasionally the unprotected young of a herd. This natural culling ensures the survival of only the fittest and thus ultimately strengthens the elk and moose herds. Bob Hohmann in the journal Wolfguard! reports that the deer killed by wolves have already been weakened by "arrows, gunshot wounds, broken bones, or disease. If diseased animals were not killed by wolves, they would breed and the herds would become weaker" (Hohmann 2). Moreover, without this natural culling by wolves, deer and moose would overpopulate and starvation could result. The government claim that wolves are to blame for the decimation of elk and moose is therefore false. The government is ignoring the real reasons for the decline of those herds. These reasons were, in fact, published in some of its most recent studies. Those studies assert that the decline of the ungulate (elk and moose) population in the last decade is directly attributed to three factors. First, the uncommon severity of recent winters up north has diminished the food supply of these animals and killed off great numbers of them. Secondly, the encroachment of civilization and industry has forced elk and moose out of their natural habitats and into unfamiliar, inimical territory. The third main reason is the trophy hunters themselves who have been "bagging" elk and moose for decades.
The end paragraph puts all you have said into perspective. Because it is your ending, your last paragraph should be charged with conviction, leaving the reader fully convinced of the validity of your argument. The very last sentence is called "the clincher" which gives a final dramatic touch to your essay. Here is the end paragraph of the wolf-kill essay:
Some people say we shouldn't concern ourselves so much with non-human issues, when there is so much human suffering in the world. But life is life and if we exterminate any part of it, we exterminate a part of ourselves. We lose an innocence vital to our existence. We are part of a master plan, a piece of a puzzle, and if we allow another part or piece to be lost, we may have to spend an eternity trying to find it again.
The end paragraph should give a sense of satisfying completeness. It should make the reader feel that you have adequately proved your thesis. It may restate the main points of your thesis without sounding obviously repetitious.