Macbeth and Banquo find some witches who tell them of their future, Macbeth shall be Thane of Cawdor, then King of Scotland and Banquo's future children shall reign later as well.
Soon, Macbeth is promoted to Thane of Cawdor after the old one is hung, leading his wife, whom he admires and loves greatly, to believe the prophecy and crave it more. Her greed leads her to convince her husband to invite King Duncan over for a meal and then, kill him.
As the King sleeps, Macbeth changes his mind about killing him. Lady Macbeth becomes angry and greedier than ever, calling her husband a coward does the trick, convincing him to retry. A dagger in his mind leads him to the Kings room, and soon, Duncan is dead.
Macduff found Duncans body the next morning and suspected something was up, despite the planted daggers on the guards and Duncans sons, Donaldbain and Malcolm, fleeing to England, who instantly became prime suspects as well as the guards.
Macbeth was crowned King and ruled with his wife, determined to remain on the thrown. Duncans sons had fled, but Banquo and his son, Fleance, were still in the picture, and not only was Banquo suspicous, but also would raise the next King. Macbeth needed him and his son gone.
So, Macbeth hired two men to have Banquo and Fleance killed, but only managed to complete half of their task, Fleance escaping to England while his father was the only one to die. Annoyed, Macbeth made sure both men were killed as well.