Always begin with a topic sentence which states what theme/concept/aspect of the text you will be discussing in the paragraph. After this, explain this theme/concept/aspect in further detail, drawing in contextual information if relevant to your argument. Then, introduce a textual example to support your argument and identify the techniques the composer uses to demonstrate their effect on meaning. Repeat as required. End with a concluding sentence that summarises your key point in the body paragraph. Follow this structure for however many body paragraphs you have.
Advice: If your 'sentence' is a dependent clause, or it doesn't contain both a subject and a predicate, then it is not a proper sentence. You can often detect fragments if you read your writing backwards sentence by sentence, . from the last sentence to the first one. You can usually correct a fragment by connecting it to the sentence before or after it.
Good writers, who have a full understanding of the sentence, occasionally choose to write a sentence fragment. So you may see sentence fragments in the fiction or even some of the non-fiction you read. As an ESL student, however, you should avoid fragments (except when writing your own creative stories).
 According to Paul Ratsmith, the tenuous, but nonetheless important, relationship between pumpkins and rats is little understood: "While I've always been fascinated by this natural kinship, the connection between pumpkins and rats has been the subject of few, if any, other studies" (2008).  Ratsmith has been studying this connection, something he coined "pumpkinology," since the early 1990s. He is most well-known for documenting the three years he spent living in the wild among the pumpkins and rats.  Though it is a topic of little recent interest, the relationship has been noted in several ancient texts and seems to have been well understood by the Romans. Critics of Ratsmith have cited poor science and questionable methodology when dismissing his results, going so far as to call pumpkinology "rubbish" (de Vil, 2009), "stupid" (Claw, 2010), and "quite possibly made up" (Igthorn, 2009).  Despite these criticisms, there does appear to be a strong correlation between pumpkin patches and rat populations, with Ratsmith documenting numerous pumpkin–rat colonies across North America, leading to the conclusion that pumpkins and rats are indeed "nature's best friends" (2008).