There are many significant actions that take place during Acts one and two in Macbeth by Shakespeare.
The first significant act is when the witches approach Macbeth and Banquo to tell them their future. They tell them that Macbeth will become Thane of Cawdor and finally king. They then tell Banquo that he will never be king but his descendants will be. This action is significant because it is the catalyst that creates Lady Macbeth and Macbeth's power hungry actions as the story progresses. Whether or not their prophecy is truth or self-fulfilling isn't important, what is important is that it causes Macbeth's downfall.
The next significant act is Lady Macbeth persuading Macbeth into killing King Duncan when he comes to visit their home. Lady Macbeth succeeds in convincing and planning, and the murder is committed at the beginning of Act 2.
The murder is of course a significant action because it turns the kingdom upside down as Macbeth has violated the great chain of being. He kills the king who is a direct link to God according to the culture of the time. This act throws of the human world as well as the natural world. Duncan's death causes Donalbein and Malcolm to be fearful, and Malcolm says "Where we are, / There’s daggers in men’s smiles; the nea’er in blood, / The nearer bloody.”" The two sons of Duncan flee to other countries. This act allows Macbeth to call their quick departure guilt and take the crown.
Witches give predictions to Macbeth, Banquo demands predictions as well. Thane of Cawdor is found to be a traitor, Duncan (the king) gives his title to Macbeth (foreshadowing and the witches' prediction come true). Macbeth thinks of killing Duncan to become king, but instantly rejects the idea. Lady Macbeth receives a letter from Macbeth telling about the prophesies. She thinks of killing Duncan, who has decided to visit the Macbeth's castle, but fears Macbeth will be too weak to do it. She convinces Macbeth that he must kill Duncan.
Banquo questions Macbeth who claims he has given no thought to the prophesies, then he imagines a bloody dagger floating in the air which leads him to Duncan's chamber. Lady Macbeth has already drugged the guards and gotten them drunk. After Macbeth kills Duncan (off stage) he returns with a bloody dagger in hand, Lady Macbeth returns it to the guards and covers them with blood to make it look like they killed Duncan. There is a knocking at the door which leads to the comic relief (tension breaker) Porter scene; the drunken porter pretends to be opening the gates at Hell. Macduff and to wake Duncan, Macduff discovers the king murdered. Macbeth, in an apparent fit of rage kills the guards. Duncan's sons Malcolm and Donalbain decide they may not be safe so flee to England and Ireland. A few days later Ross and an old man discuss the supernatural happenings when MacDuff arrives to report the suspicions against Macbeth (for killing the guards) and Ducnan's two sons (for fleeing), but notes that Macbeth will become king. Ross goes to Scone for the coronation but MacDuff instead heads home (hinting at his distrust of Macbeth, and foreshadowing future conflict).