Citations are always made of pieces of information about your chosen sources which provide your readers with the ability to find and read them as well. The elements of a citation answer four basic questions about a source: Who (wrote/performed/edited it), What (are its titles), When (was it published), and Where (did you find it).
Basic Citation Elements:
When it comes to books, the title page and copyright page--often the front and back of a single page--are the best places to find citation information. Other places like dust jackets, book covers, and indexes will often omit or add information you should not use in your citations.
Scholarly journal articles usually add the citation information you need to the first page, often including the abstract. A hosting database may also add some or all of this information to a downloaded article's last page. However, be cautious about using that information because it is computer generated by the database and has not been reviewed by the publishing journal.
Unfortunately, there is no uniform standard for citation information placement in other types of sources. Examine every source you use to find its author, title, and publication facts. Ask a librarian for assistance if you have questions!