the poet to whom?i mean the poet taking to the readers or himself?
The poet is narrating his journey through the woods for the reader so he is talking to the reader. However, there is an element of him having a dialogue with himself. In the line one he says, " Whose woods these are I think I know." and he goes on to develop what could be an internal thought about how the owner of the woods will not know he has been there. He also puts his thoughts about the journey through the woods onto the horse. He says his little horse "must think it queer/to stop without a farmhouse near" (lines 5-6)
Additionally at the end of the poem the poet also is reminding himself of the "promises" he has to keep. This also has an element of an internal discussion but it is also a part of the narration to the reader.
So in answer to your question, both.
THe narrator is talking to himself, his horse, and the reader. Anytime you speak aloud, others can hear it, whether these thoughts are directed to them or not.
He speaks about the journey, the snow, the beauty of the surroundings, the house in the woods, the eminent destinations (promises to keep), and the horse's thoughts about the stopping in this spot.