In the book The Host by Stephenie Meyer, when it says 'I' who is speaking?

Your question has to do with point of view. You are right
in identifying that the story is told from the first person perspective, using the word
"I" to relate the story as told from the point of view of one of the central characters.
If we read the story carefully, we see that the story is told from the point of view of
the Wanderer. What confuses the narration is the way that the Wanderer inhabits Melony's
body with Melony's emotions and feelings still being a force against which the Wanderer
struggles against. However, at no stage in the narrative is the story told from
somebody' elses perspective. It is the Wanderer that dominates the narrative, whatever
body she has chosen to sojourn inside.