Some sources may have an editor instead of an author, in which case you often use the editor's name in place of the author's. In some cases the author may be an organization rather than an individual. If there truly is no author, follow the guidelines for the citation style you're using (summarized below).
MLA style: In the Works Cited entry, leave out the Author field. In parenthetical citations, use a shortened version of the title in place of the author's name.
APA style: In the Reference list, move the title of the work to the beginning of the entry. For in-text citations, either introduce the work by its title in a signal phrase or use a shortened version of the title (1-2 words) in place of the author's name in parentheses.
Chicago style: Start the citation with the title of the work instead of the author. If using author-date style, use the title in quotation marks in the parenthetical citation.
AMA style: Leave out the author field.
CSE style: In the name-year system, move the title of the work to the beginning of the reference list entry. Use the first few words of the title followed by an ellipsis (...) in place of the author's name for in-text citations. In the citation-name and citation-sequence systems, leave out the author field.
