When creating a thesis about The Chronicles of Narnia, you would be best served in finding some overarching idea about the work that you could present through evidence readily available in the text. This could be anything, though one of the easiest ideas that you could explore is the applied allegorical lens through which the work can be viewed.
C. S. Lewis himself claimed that the work was not allegorical by intention. However, the parallels with Christian theology and mythology can be seen quite clearly. For example, the character of Aslan has widely been regarded to be influenced by the messianic figure of Jesus Christ. His sacrifice in the second installment of the series, The Lion, the Witch, and the Wardrobe, holds many parallels with Christ's sacrifice for all of mankind, as well as his subsequent rebirth. The Magician's Nephew tells the story of the creation of Narnia and very explicitly portrays an intelligent creator dictating the land into existence.
Conflicts within the world revolve around the biblical canon's idea of good and evil, and oftentimes antagonists act out of spite or seem simply possessed by a compulsive drive to commit evil due to vanity or arrogance—similar to the biblical antagonist Satan.
With all this in mind, a possible thesis when attempting to deliver this idea might read as follows:
Through careful comparison with the Bible, as well as through a study of C. S. Lewis's life, parallels can be drawn between the characters, events, and ideals of The Chronicles of Narnia and those of the biblical canon.