Confused about Transitional Sentences

          As I read through a memoir in chapter 60 of my English 101 course book, I see the paragraphs of the article jump from one topic to another without really relating to the same thing.  The first paragraph starts off by explaining a little bit of the author’s history and the thesis statement which is related to making friends.  The next paragraph starts off with describing another area that he had lived at as a boy.  I see this article going from subject to subject and apparently, the article is correct.  I do the same thing when I write and I get marked down…..I obviously do not understand transitional sentences.