The first 3 scenes set the stage for the whole tragedy. The weather is stormy, which hints that there'll also be a tumult of feelings in the play. The witches are also part of the nature - they appear in the storm, they rule the winds and they crash people's lives. Meanwhile the situation in Scotland is also far from calm - the king of Norway together with one treacherous thane rise against the rightful king. They are defeated, not without the help of Macbeth, and the title of the traitor - the thane of Cawdor - goes by king's wish to Macbeth. Inheriting the traitor's title doesn't mean anything good, eh?
The most powerful scene is the meeting of Macbeth and the witches. They greet him with the title he knows he has, with the title he doesn't yet know he has (the thane of Cawdor) and with the king's title. To make things worse the moment later appear two noblemen, who bring him news of his new Cawdor title. Enough to believe in the whole prophesy, and Macbeth, secretly wishing to be the king, never doubts it.
Now, is it a predestination and do witches really see the future? I think not. I think the decision is still to be made by Macbeth. In scene 3 he still hopes that the prophesy will come true without his help, but surely he must know the old king must be dead to have a new one? So here is the moment where the thought is planted in Macbeth's mind, and we'll see if it's enough to make him a traitor too, as his predecessor was.
Act I, scenes 4 - 7
In these acts we finally see who wears the pants in Macbeth's household. Lady Macbeth is the one who really makes the treason happen. She knows her husband is too soft and true, and, it seems to me, detests him because of this. She understands that never again will she have such a perfect chance to become a queen - her husband is convinced that this must happen, and the king is going to spend the night at their castle. She also realises that she is to guide her husband and organise everything, because he is ready to stop at any moment, as, despite his ambition, he likes the king, he is his liege and he is recently rewarded by him. But what can he do if she has a perfect plan, and she uses her woman's jedi-tricks, including "you are not a man" and "you don't love me anymore? See how she prays for the strength to accomplish the deed:
....Come, you spirits
That tend on mortal thoughts, unsex me here,
And fill me from the crown to the toe top-full
Of direst cruelty! make thick my blood;
Stop up the access and passage to remorse,
That no compunctious visitings of nature
Shake my fell purpose, nor keep peace between
The effect and it! Come to my woman’s breasts,
And take my milk for gall, you murdering ministers,
Wherever in your sightless substances
You wait on nature’s mischief!
Gives me shivers! But although she terrifies me and I can't approve of her readiness to kill, somehow I also admire her, because she is so strong and determined.
So act I is finished, and I am already thrilled! The murder hasn't happened yet, but it is there from the first lines, and the growing anticipation makes the first act such a brilliant piece of writing.