During the meetings between the witches and Macbeth, the witches told him what he wanted to hear. The put ideas into Macbeth's head about having the power to become king. The witches knew that Macbeth would act upon the thoughts which they gave to him.

Act 1, Scene 3

First Witch
"... and munch'd, and munch'd, and munch'd: 'Give
    me,' quoth I.
'Aroint thee, witch!' the rump-fed ronyon cries.
Her husband's to Aleppo gone, master o' the Tiger
But in a sieve I'll thither sail, And, like a rat without a tail,
I'll do, I'll do, and I'll do.

Second Witch
I'll give thee a wind,

First Witch
Th'art kind.

Third Witch
And I another.

First Witch
I myself have all the other;
And the very ports they blow,
All the quarters that they know
I' the shipman's card.
I'll drain him dry as hay:
Sleep shall neither night nor day
Hang upon his pent=house lid;
He shall live a man forbid.
Weary sev'n-nights nine times nine
Shall he dwindle, peak and pine:
Though his bark cannot be lost,
Yet it shall be tempest-tost.
Look what I have.