Basic Essay Structure 

In many languages, the fundamental unit of composition is the paragraph.  A paragraph consists of several sentences that are grouped together.  This group of sentences ONLY discusses ONE main subject.   In English formal academic language, paragraphs have three principal parts.  These three parts are the topic sentence, body sentences, and the concluding sentence. 

A topic sentence usually comes at the beginning of a paragraph; that is, it is usually the first sentence in a formal academic paragraph.  (Sometimes, this is not true, but as you practice writing, please keep to this rule unless you are instructed otherwise).  Not only is a topic sentence the first sentence of a paragraph, however, more importantly, it is the most general sentence in a paragraph.  In other words, there are not many details in the sentence, but that the sentence introduces an overall idea that you want to discuss later in the paragraph. 
 For example, suppose that you want to write a paragraph about the natural landmarks of your hometown.   The first part of your paragraph might look like this:
My hometown, Nhatrang, is famous for several amazing natural features.  First, it is noted for the Cai River, which is very wide and beautiful. Also, on the other side of the town is Red Hill, which is unusual because it is very steep

The first sentence begins with "My hometown..." a few spaces to the right of the paragraph edge.  This is an indentation.  All paragraphs in English MUST begin with an indentation.)
The first sentence, My hometown, Nhatrang, is famous for several amazing geographical features, is the most general statement. This sentence is different from the two sentences that follow it, since the second and third sentences mention specific details about the town's geography, and are not general statements.

The problem with sentence #1 is that it contains too many details. Topic sentences are general, and details should appear later in the paragraph. A better topic sentence would be like the one mentioned above, My hometown is famous for several amazing geographical features.
The problem with sentence #2 is not appropriate as a topic sentence because it mentions two topics, not just one. Paragraphs are usually about one main thing and so their topic sentences should also be about only one main thing.
The problem with sentence #3 is that it is too general. It is also very boring! Would you like to read a paragraph with this topic sentence? Most people would not.
We can rewrite sentences #2 and #3 in the following ways to make it better:
There are two reasons why some people like to buy cars with automatic transmission.
OR (in a different paragraph)
There are two reasons why some people like cars with manual transmission.
The shapes of clouds are determined by various factors.

Consider again the above-mentioned, short paragraph:
My hometown, Nhatrang, is famous for several amazing natural features.  First, it is noted for the Cai River, which is very wide and beautiful. Also, on the other side of the town is Red Hill, which is unusual because it is very steep
When a reader reads a topic sentence, such as My hometown, Nhatrang, is famous for several amazing natural features, a question should usually appear in the reader's mind.  In this case, the question should be like, "What are the natural features that make Nhatrang famous?"   The reader should then expect that the rest of the paragraph will give an answer to this question. 
Now look at the sentences after the topic sentence.  We can see that the second sentence in the paragraph,  First, it is noted for the Cai River, which is very wide and beautiful, indeed gives an answer to this question. That is, the second sentence gives some explanation for the fact that Nhatrang is a famous town. Similarly, we can see that the third sentence also gives some explanation for the fact that Nhatrang is famous by giving another example of an "amazing natural feature," in this case, Red Hill.
The second and third sentences are called supporting sentences.  They are called "supporting" because they "support," or explain, the idea expressed in the topic sentence.  Of course, paragraphs in English often have more than two supporting ideas.   The paragraph above is actually a very short paragraph.  At minimum, you should have at least five to seven sentences in your paragraph. 
Here we can see our paragraph about Nhatrang with a few more supporting sentences in bold font:
 
My hometown is famous for several amazing natural features.  First, it is noted for the Cai River, which is very wide and beautiful. Also, on the other side of the town is Red Hill, which is unusual because it is very steep. The third amazing feature is the Big Old Tree. This tree stands two hundred feet tall and is probably about six hundred years old.

In formal paragraphs you will sometimes see a sentence at the end of the paragraph which summarizes the information that has been presented.  This is the concluding sentence.  You can think of a concluding sentence as a sort of topic sentence in reverse.
You can understand concluding sentences with this example.  Consider a hamburger that you can buy at a fast-food restaurant. A hamburger has a top bun, meat, cheese, lettuce, and other elements in the middle of the hamburger, and a bottom bun. Note how the top bun and the bottom bun are very similar.  The top bun, in a way, is like a topic sentence, and the bottom bun is like the concluding sentence.  Both buns "hold" the meat, onions, and so on.  Similarly, the topic sentence and concluding sentence "hold" the supporting sentences in the paragraph.  Let's see how a concluding sentence might look in our sample paragraph about Nhatrang:
My hometown is famous for several amazing natural features.  First, it is noted for the Cai River, which is very wide and beautiful. Also, on the other side of the town is Red Hill, which is unusual because it is very steep. The third amazing feature is the Big Old Tree. This tree stands two hundred feet tall and is probably about six hundred years old. These three landmarks are truly amazing and make my hometown a famous place.