Paragraphs in non-fiction documents typically begin by starting off with a topic sentence introducing what is being discussed. The topic sentence is usually indented with a tab. Following this sentence, two to three explanatory sentences that elaborate on the idea, provide evidence, etc... Finally, a conclusion statement is given to end it.
In non-academic use, paragraphs seldom have to be that formal in structure; they're simply a way of dividing text into easier to read segments. Paragraphs don't have to be any particular length, as long as it makes sense for them to be that long.