Perhaps the first thing you should do is consider how long this essay is going to be; are there guidelines about the length? An author's life story alone would probably be too broad of a topic to tackle in an average sized essay, but then to add a discussion of the author's work, and even critical reviews of the author's work could, depending on the author, take your "essay" into a full-length book, or multi-volume set! Again, it depends on the author, of course, but if you choose to go the route of biographical information, it may be necessary to focus on one area of the author's life, rather than a birth to death type thing; if you focus on his or her work, it may be necessary to focus on one piece, or maybe do a compare/contrast on two pieces. For example, one might compare the depiction of the Roaring Twenties in two of the novels by F. Scott Fitzgerald. If you were to decide to go in the direction of critical analysis, you might do well to focus on different critical commentaries about one of the author's pieces.