The topic sentence state the main idea of the the paragraph .
There are many ways in which one can do this.
Some possible techniques:definition /analysis/
statistics/comparison and contrast /and cause and effect.
The topic sentence need to be the most general sentence of the paragraph.
meaning that it doesn't need to have too much detail or too much "fluff"
The following sentences will then expand on the topic sentence.
However does need to establish the point of the paragraph so that the reader con get a good idea of what the paragraph will include, Just from the topic sentence.