I teach students how to write essays. Often students struggle with essays because they don't understand essay structure.
Think of essay writing as architecture. To build a house, you follow codes that ensure a basic build quality. A house that meets this standard, won't fall over, let the rain in, or let the heat out. It might only be a bungalow, but at least it will stand.
Essay writing is the same. By following good rules, you ensure a good argument. A structurally sound argument will also be sound argumentatively because a good structure will prevent you from writing a bad argument.
It will actually improve your ideas.
Introduction
Your introduction should include three levels of focus: topic, subject, and thesis statement. Your topic is your broad field of study, your subject is a specific area in that field, and your thesis is a specific argument about your subject.
Think of these like a funnel that begins broadly before closing in. You need not be so blunt as to say, "My topic is [blank] and my subject is [blank]," but identifying your topic and subject first will provide context for your thesis.
For example, if your topic is Star Wars, then your subject could be racial representations in Star Wars. Your thesis, then, could be "George Lucas's dependence on racialized stereotypes in the prequel trilogy substitutes good character development."
To begin your essay, you should first give a sense of race in Star Wars: "While the Republic and the Rebellion in Star Wars are predicated on tolerance and racial parity, the films themselves do not always demonstrate the same." This sentence establishes context and focus for the thesis to follow.
Thesis
Your thesis statement is the most important part of your essay. It is the crux of everything and functions as a guide to your entire paper.
This means there should be nothing in your paper that isn't at least indicated by your thesis, and the rest of your paper should constitute a series of supporting arguments and evidences for your thesis.
A good thesis is a single sentence (two at most) that states your argument clearly and argumentatively. Your thesis must also make a claim on your subject. This means it must be debatable (something which can be argued against) and not a mere statement of fact.
It should also be specific. Your thesis determines your paper's focus, so you want it to be on point.
As a good test, I tell my students they should never be more than one sentence away from their thesis. By this, I mean that if, at any point in their essay, they need more than one sentence to connect what they're saying back to their thesis then they are off-track.
Where you place your thesis in your introduction is up to you. Generally, I don't recommend placing it first, where context should go, nor in the last sentence.
Find the logical place for your thesis. Sometimes this means placing it early so the rest of introduction can expand and provide supporting arguments. Other times, you might find you need other arguments before your thesis.
Body-Paragraph Structure
The body makes up the largest part of the essay. There are two levels of body paragraph structure: how each paragraph is organized within itself and how the paragraphs are organized in relation to each other.
I urge a five-part internal paragraph structure. A sufficient body paragraph needs at least five sentences. Each paragraph must include five specific elements in sequence:
A topic sentence
Elaboration on that topic sentence
Evidence
Discussion of that evidence
A transition sentence
A topic sentence is a thesis statement for your paragraph. It provides the specific argument the rest of the paragraph will prove. Because your topic sentence is so focused, it requires some elaboration, so your second sentence needs to expand on the first.
While your topic sentence should be limited to a single sentence, your elaboration can be longer. Together, these two or more sentences form your full statement of argument.
Every argument also needs evidence. Evidence can come in many forms—textual evidence (quotation, paraphrase, or summary), statistics, expert opinions, testimony, specialized knowledge, physical observations, etc.
Before you use evidence, though, you need to set it up. This can be a simple attribution ("Bill Simpson says") or a sentence that establishes ideas in the evidence. Ultimately, you need to provide sufficient context such that your reader is prepared to encounter the evidence.
Once you provide your evidence, you need to discuss it. Never assume your evidence is self-evident. You must show how your evidence proves your argument. This means fully discussing the implications of your evidence and connecting it back to your topic sentence.
Finally, you need to provide a transition sentence. This sentence does exactly what you think: It wraps up the paragraph and prepares the reader for the next.
A transition sentence is a bit of an art form. As a segue, it bridges discussions in two different paragraphs. How it does that is up to you but it needs to do so sufficiently.
A transition sentence can conclude a paragraph in a number of ways. It can summarize the paragraph, connect the paragraph back to the thesis, or indicate how the next paragraph will follow. Ideally, it will do all three.
Body Organization
As a writer, you also need to organize your paragraphs overall. An essay should be organized according to an internal logic wherein one idea leads logically to the next.
The importance of this is already implied by the transition sentence—an essay with a logical progression will make transitioning between ideas relatively simple.
There are also bad ways of organizing an essay.
Someone has probably suggested you should begin with your strongest idea then move to your lesser ones. You may have been told the opposite. You might also have heard your lesser arguments should go in the middle so you can begin and finish strong.
These organization models all have the same basic problem: they include space for weaker ideas and hinge upon those ideas for their organization.
You should never have weak ideas—your paper is better short than weak. The above models have no way of organizing a paper because they're predicated on the relative effectiveness of paragraphs instead of the ideas those paragraphs convey,
Organizing your essay means identifying the separate functions of each paragraph and understanding how each function fits into the essay overall. Each paragraph should have a separate purpose, just as each sentence has a separate function.
To eliminate redundancy, every paragraph should advance the essay further than the last.
You'll find that having some paragraphs before others just makes sense. This is because some ideas are more preliminary while others build on ideas that need to come before.
Good overall organization depends on your ability to know and craft a separate intent into each paragraph. With this, organization is as simple as determining a logical sequence.
An essay built on such logic will be harder to attack. If each separate argument fits tightly into an overall argument then attacking one idea means attacking them all. This is harder to do than criticizing discrete arguments that do not build on each other.
Conclusion
The last section of an essay (except for references) is the conclusion. Conclusions need not be long arduous rearticulations of everything you've said. They can simply provide the final idea your paper leads up to (this is especially effective if your paper has a good logical sequence).
While your conclusion should give some indication of the paper's arc, it should more importantly focus on the take home idea you want your reader to have.
Your conclusion can also connect your paper to a larger discussion in your field or recommend further study. You began your essay by contextualizing it and you can do so her too.
Perhaps your paper exemplifies a larger thematic discussion or perhaps it should but that larger discussion doesn't exist yet. Either way, you can connect your discussion to others, demonstrating the larger importance of your specific argument.