How did Macbeth gain power in Scotland?
At the beginning of the play, Macbeth is already a hereditary lord, or thane, or Glamis. We learn that he inherited this position from his father, Sinel. He becomes Thane of Cawdor because of his valor in battle, and because that man treacherously joined the king of Norway in an attempt to conquer Scotland. We also learn that Macbeth is a cousin of Duncan, and next in the line of succession behind the king's sons Malcolm and, it seems, Donalbain. The point is that by birth, Macbeth is situated at the highest levels of Scottish society, and that he has seemingly improved his status through his performance in battle. But he is not quite as powerful as he would like to be once he hears the witches' prophecy, and so he (and his wife) conspire to murder the king. When they do so, and the king's sons flee to England to avoid their father's fate, he is next in line to the throne as Duncan's kinsman, and so he is crowned.