To write a topic sentence for a paragraph, choose the main idea of the information in the paragraph and summarize the information into a sentence. The topic sentence should be general or broad enough to encompass the overall subject of the paragraph, according to Purdue University's Online Writing Lab. A topic sentence should also be specific enough to hone in on the supporting argument or idea presented within the paragraph.
Even though the topic sentence should not be too general or specific, it should be written in active voice and with direct language. Topic sentences provide focus for the paragraph and help the reader to understand the information communicated. By detailing the main point of the paragraph, the reader is better prepared to absorb the paragraph's contents, which may include facts, details, personal accounts, anecdotes or supporting arguments.
For example, if the paragraph is about how the death penalty is more expensive than housing a prisoner for a life sentence, the topic sentence should generally state that the death penalty is not cost effective for the government and tax-paying citizens. This general idea as the topic sentence can now provide the opportunity for the writer to detail the specific expenses, statistics, facts and details to support the overall claim in the general topic sentence.