Macbeth becomes king with the help of his wife, murdering King Duncan and framing his guards (and killing them too, for good measure).
Although King Duncan's eldest son Malcolm was declared heir, his fleeing incriminated him for his
father's murder, as with his brother Donalbain.
As a high-ranking thane (of both Cawdor and Glamis), and being a cousin of King Duncan, Macbeth is selected as his successor.
How he keeps his throne is a story involving more blood and betrayal.
Hope it helps!
