If the theme you use does not provide an author byline then you either
(1) type it on in on the first line in the body of the text of the posts or
(2) or use this workaround for a single author
Regardless of the theme, bylines will only display now if there are at least two authors who both have at least one published post in the blog.
(3) you switch themes and use one that does display author bylines http://wpbtips.wordpress.com/2009/08/15/author-displayed-or-not/