It is in Chapter Twelve that the creature relates how he learnt to talk. This of course comes during his conversation with Frankenstein on the glacier and occurs during the creature's stay with the De Lacey family. It is through observing and listening to them that the creature learns the rudiments of speech. Note what he tells his creator about the process:
"By degrees I made a discover of still greater moment. I found that these people possessed a method of communicating their experience and feeligns to one another by articulate sounds. I perceived that the words they spoke sometimes produced pleasure or pain, smiles or sadness, in the minds and countenances of the hearers. This was indeed a godlike science, and I ardently desired to become acquainted with it."
It did not come easy, but through application and lots of hard work, the creature was able to learn how to speak through listening in on the conversations of this family and thus himself was able to master this "godlike science" of communication. Thus it is that he is able to communicate to Frankenstein at this point in the novel.