A paragraph consists of several sentences grouped together, all of which discuss one topic. The first sentence in your paragraph is the topic sentence. It is the most general sentence in the paragraph and introduces an overall idea. It should suggest a question(s) in the reader’s mind and point your reader in the direction you are going; it does not provide any detailed information about the idea, It is usually the first sentence in the paragraph and should be indented. It can also make a reference to the preceding paragraph. The next sentences in a good paragraph are the supporting sentences, They should answer the question(s) suggested in the topic sentence and provide explanations (facts, details, examples) that support or explain the main idea expressed in the topic sentence. You can include as many sentences as necessary to accomplish the explanation. The concluding sentence summarizes the information presented within the paragraph. It is similar to, but not exactly the same as, the topic sentence.
A good paragraph includes four elements: unity, order, coherence, and completeness. To achieve unity you need to maintain one controlling idea (a single focus). This main idea is expressed in the topic sentence, detailed in the supporting sentences, and summarized in the concluding sentence.
Order can be cause & effect wherein a situation either causes or results from another, chronological or order of events in time, comparison/contrast or similarities and/or differences among things, emphatic, which means details are arranged in order of importance or for emphasis, and/or spatial, which is the discovery of how things are arranged in a space. Coherence is the element that makes a paragraph understandable. It uses logical bridges; in other words, the same idea is carried from one sentence to the next and successive sentences are constructed in parallel form. Verbal bridges can be used as well (e.g., pronouns refer to nouns in previous sentences, keywords are repeated in several sentences, synonymous words are repeated in several sentences, transition words link ideas from different sentences). Consistent verb use and point of view are maintained throughout. To achieve completeness you must maintain a single idea throughout the paragraph.
One of the most prominent questions when developing good paragraphs is, "When do I start a new paragraph?" The answer is simple: 1) when beginning a new idea, 2) to contrast ideas or information, or 3) when your reader needs a pause. Bear in mind that paragraph construction involves taking liberties when writing dialogue, and than's a whole other ball game. We'll cover that another time.