It is beyond the scope of this document to treat paragraphing in detail. Here I content myself with a few brief remarks.
Every piece of written work should be broken up into a series of reasonably small paragraphs, and each new paragraph should represent some kind of break, however small, in the continuity of the text. Some people have trouble with this, and tend to produce enormous paragraphs running to a whole page or more. This is very tiring for the reader and should be avoided. If you have this kind of problem, try studying the paragraphs in any longish section of this document; this may help you to get a grasp of where it is appropriate to start a new paragraph.
As remarked above, the first paragraph after a title or section heading in business letters, there is another format which is sometimes preferred. In this second format, every paragraph is separated from the next by a blank line, and no paragraphs are indented. This format uses more paper, and it is not normal in other types of writing.