In Act 1, Scene 3 the witches prophesize that Macbeth will be Thane of Cawdor and then the king. These are the first two prophecies. The exchange is quick:

W1: All hail, Macbeth! Hail to thee, Thane of Glamis!

W2: All hail, Macbeth! Hail to thee, Thane of Cawdor!

W3: All hail, Macbeth, that shalt be king hereafter! (I.iii.51-53)

At the time of this prophecy, Macbeth is already the Thane of Glamis, so the first real prophecy is that he will become the Thane of Cawdor. This, however, is confusing to Macbeth because, as he says, "But how of Cawdor? The Thane of Cawdor lives / A prosperous gentleman, and to be king / Stands not within the prospect of belief, / No more than to be Cawdor" (I.iii.75-78). Yet, it is soon revealed that the Thane of Cawdor committed treason, ensuring that Macbeth becomes the Thane of Cawdor. This initial prophecy comes true, causing Macbeth to wonder how he will next become king.