To write a good topic sentence, the writer must identify the main idea of the paragraph. This must be balanced with being general enough to include the entire paragraph and supporting details.
Ideally, the topic sentence identifies the main idea, or point of the entire paragraph. The other sentences in the paragraph are called supporting details, and should support the main idea of the topic sentence. Supporting details further expand on, or explain the main idea presented in, the topic sentence in greater detail.
A well-organized paragraph supports one controlling idea. The controlling idea is the topic sentence. The topic sentence should unify the details given in the paragraph. A good topic sentence supports the thesis of a paper while also tying in the supporting information. Most often it is appropriate to begin a paragraph with the topic sentence, since this is the area readers look to first as they figure out what the paragraph covers.
Most paragraphs have a structure of an introduction, body and conclusion. An introduction is the first section of a paragraph, and should include the topic sentence, as well as sentences that give background information or provide a transition. The body follows the introduction, and uses supporting sentences to discuss the controlling idea, show facts, examples or other information. The conclusion is the final section. It should summarize the topic sentence and the paragraph's content.