What is Macbeth's initial reaction to the witches' prophecies and what causes a change in his attitude

Answer: He is excited at the prospect of becoming king but also fearful about what role he will have to play in making it happen

Macbeth initially rejects the witches' prophecy as meaningless. He doesn't believe it.

It's only when one of their predictions, that Macbeth was to become the thane of Cawdor, comes true, that Macbeth believes the prophecy of the witches, thus putting in motion the plot of the play (Macbeth trying to fulfill the prophecy that he is to become the king).