who begins narrating the story, and what is her first impression of bly?

If you mean the prologue, it is told by an anonymous narrator who seems educated and of the upper class.

The narrator makes it clear that some of the guests are more sophisticated than others and that those who remain to hear Douglas’s story are a select group. This group is characterized as “arch,” meaning deliberately or even forcedly ironic and playful. The group’s members are fairly aggressive about reading between the lines of what Douglas says to draw sexual inferences, as Mrs. Griffin does about Douglas and the governess. The guest who wisecracks about the former governess dying of “so much respectability” is insinuating that the former governess is less than respectable, perhaps morally and sexually loose. This guest treats Douglas’s story skeptically, even cynically, refusing to take things at face value and ready to make inferences of a sexual nature. When we learn more about this former governess—Miss Jessel—we see that this guest is absolutely right and perhaps conclude that we are intended to read The Turn of the Screw in the same way: viewing the characters realistically rather than romantically, treating the story skeptically, and reading between the lines for sexual overtones.

If you mean the governess who narrates the rest of the novel, you can find more at the link below.