Macbeth and Banquo run into three witches. They tell Macbeth that he will become Thane of Cawdor, a king, and the King of Glamis. They tell Banquo that he will be the father of a king.
Macbeth sends a letter to Lady Macbeth informing her of the witches' prophecy, that he has become Thane of Cawdor, and that Malcolm stands in the way of him becoming King.
Macbeth and Lady Macbeth are generous hosts, even though they plan to kill King Duncan. During dinner, they make sure to get Duncan's guards very drunk so that they will pass out and leave Duncan unprotected.
When Duncan and his guards are sleeping, Macbeth prepares himself to kill Duncan. He has a moment where he is confused and doesn't want to kill Duncan, but then decides that he must go through with the plan.
Macbeth decides that this must be swift and quiet. He slips a pillow out from underneath a drunken guards head. He plans to hold the pillow over Duncan's mouth to mute any noise that might erupt out during the attack.