When interpreting a text, you must deal with what you do know.
What do you know? You know the text itself; indeed, this is the only thing you know in its entirety.
You can know how the work was published and know something about how it was received and how it has been used since publication.
You should know something about it's time, place, and context. You can't make major assumptions about how these aspects contribute to the work--you must focus on the text--but this can help to make sense of the text.
You have the author's other works, and possibly the things the author has written/said about the text you are examining. The other works can be helpful in making sense of a particular work; however, you have to be somewhat skeptical about what the author says/writes about the work. Factors such as time, reactions to the work, and evolving self-perception taint those comments.
You may know something about the author's worldview that can help you make sense of the text.
You know next to nothing about the author's intent; even if the author speaks of his/her intent, you cannot depend on that for certain.
You know next to nothing about how the author's biography influences the text.
You know next to nothing about psychological issues of the author, or how they influenced the text.