In order to write an essay about "Mending Wall," you need to ask yourself what it is that you would like the reader to take away about this poem after reading your essay. This poem has a great deal to say about friendship and walls, doesn't it? One question to explore is why the narrator does not like walls. Another is why his neighbor does like the wall. You might write an essay contrasting the two neighbors in the poem, who clearly see life very differently.
Whatever it is that you want your reader to take away is your thesis, your main idea, and that thesis must be supported by the text of the poem. So, for example, if you were to write an essay on why the narrator doesn't like the wall, you might use the very first verse of the poem,
Something there is that doesn't love a wall,
That sends the frozen-ground-swell under it,
And spills the upper boulders in the sun;
And makes gaps even two can pass abreast (Frost lines 1-4)
You could argue that the narrator does not like the wall because nature is against it, always trying to tear it down. That is using the text to support your thesis.
As you decide upon a thesis, you will need to incorporate it into a thesis statement, one sentence that states your main idea and the points you will discuss to support that idea. So, for example, if I were writing an essay about why the narrator does not like the wall, I could have this as a thesis statement:
The narrator in "Mending Wall" shows that he does not appreciate his wall because it is against nature, it serves no purpose, and it does not make him and his fellow wall-mender good neighbors.
That states a thesis, the narrator's attitude toward the wall and three aspects of the poem that provide the reasons. Your thesis statement should be the very last sentence in your introduction.
For an essay in which you have three supporting points in your thesis statement, assuming that you do have three points, you will write a five-paragraph essay. The first paragraph will introduce the essay, including the name of the poem and its author, as well as your thesis statement. The next three paragraphs will be body paragraphs, each one discussing a point from the thesis statement, in the same order in which you "list" those points in the thesis statement. Give each of these a good topic sentence to let the reader know which point you are developing. Finally, you will have a fifth paragraph that will be your conclusion. In a conclusion, we remind the reader what the main idea is and review the points made in the body paragraphs.