To properly fulfill the mitzvah of Krias Shema, a person must recite all three paragraphs (Shema/Va’ahavta, V’haya Im Shamo’a, and V’yomer). A person must have proper concentration on at least the first two lines – the lines of Shema Yisrael and Baruch Shem Kavod, or else he must repeat the Shema. (Of course, a person should try to concentrate on all of the words.)