One of the best-known sayings in writing is "write what you know." This doesn't mean that things can't be made up in fiction, but, in order for it to be believable, the author needs to know his subject matter. So, in fiction or nonfiction, a prospective author needs to consider what she knows and what she can talk about with authority. This includes topics, people, places and ideas. An author writing on a topic that he knows nothing about in nonfiction is obviously a bad idea; but, even in fiction, not knowing the subject matter will make the story less believable and the writing much more difficult to write.
For nonfiction, such as self-help, how to, scientific or historical work, it is especially important to know the subject matter. It is necessary to thoroughly understand something before the information can be shared with readers. An author in this are should also be aware of the books on the topic that are already published, so she knows what has been covered, and what is missing. This will help to determine whether or not another book on the topic is needed or will sell well.
Whether a work of fiction is based on real people, or completely fictional, the author should know the characters, including their personalities, histories, mannerisms, likes and dislikes. This will help make the characters more believable and make the story easier to tell. Whether the story takes place in a real location, or a fictional one, the author should know the place as well as they know the characters. In order to tell a story and make readers feel like they have truly lived it, the author must feel that way first.
Plagiarism, or simply copying someone else's work, is illegal. Beyond that, a reader should never feel like they are reading a book that they've already read. Prospective authors should think about what they can bring to a story or a subject that is new, fresh and original. By determining what he or she can write about with knowledge, confidence and originality a prospective author should be on the road to finding a subject for their book.