The first five chapters (which do not include the letters) of the novel are narrated by Victor Frankenstein. He is telling his life story to Robert Walton.
The book begins with letters, however, and sometimes those are confused as being the first few chapters of the book. Those letters are narrated by Robert Walton. The letters help us to understand Walton, his background, and how he came to meet Frankenstein. Walton appears again towards the end of the novel and, in a way, acts as bookends for the story.
Shelley uses multiple narrators in the novel (Walton, Victor, and the Creature) in order to show different perspectives. For example, because Victor and the Creature feel so differently about the different actions each of them has taken, their accounts provide different perspectives that help the reader to understand all of these characters.