Shakespeare bases most of act I on the prophecies of the three witches that open the play. The first words that Macbeth speaks, "so foul and fair a day i have not seen" mimics the witches opening line, "Fair is foul and foul is fair." Already, by Macbeth's first line, the reader can tell that what the witches say will be echoed by Macbeth. The witches' dark and mysterious aura foreshadows the rest of the play. When the Witches tell Macbeth that he will be Thane of Cawdor and King, Macbeth doesn't know whether to believe them. However, when he shortly after learns that he has become Thane of Cawdor, which the witches had said, he realizes that he also will be King. Macbeth believes it is his destiny to be King, and Act 1 after the prophesy unravels with Macbeth's internal conflict with his desire to be King. Shakespeare brings up the question of Macbeth's destiny with the witches, and shapes the act around it, using the witches to foreshadow the evil that is to come.
In addition to the witches "shaping" Macbeth's destiny, we also see a fundamental change in his character because of the witches' prophecy. While previously he is filled "with the milk of human kindness" (per Lady Macbeth), he begins to become selfish and power-hungry. While before, he would have considered being named the Thane of Cawdor an honor, now he wants more, and it is merely a stepping stone to "bigger and better" things. Finally, the biggest evidence of this change is how he agrees to follow Lady Macbeth's plot to murder King Duncan, a kind man and virtuous leader who has never wronged and only honored Macbeth.
In Act 1, this act establishes the play's dramatic premise- the witches' awakening of Macbeth's hunger and desire and ambition to conquer the world. This acts brings about a dark mood that would transpired throughout the whole play itself. When the Three Witches proclaimed that Macbeth would become heir to the throne and become Thane of Cawdor, his reaction to this announcement can tell us that he has great desire for power and prestige and to control the whole empire under his rule. He mimicked what the witches said: "So foul and fair a day I have not seen”. This line echoes the witches' words and established a connection between Macbeth and the three witches and deeply suggests that Macbeth would play a key part and a central role in the play's moral confusion and agony. This foreshadows his quest for success as he killed people like Duncan who gets in his way, to usurp the throne and control the whole empire.