Ambition and Macbeth are two words that seem to be inextricably intertwined throughout the entire play, and you would do well to trace ambition as a theme in relation to the progression of Macbeth's character in this excellent tragedy. To help start you thinking through this theme, let us look at Act I scene 3, which is of course important both because it is the first scene where we meet Macbeth and also when he receives the prophecies that cause so much trouble.
What is interesting about this scene is the way that Banquo describes Macbeth to us even before we find out about what he is thinking about the words the witches have uttered to him. Banquo describes Macbeth as being "rapt withal" with the words of the weird women. This indicates Macbeth's unhealthy sense of ambition. When Macbeth has had the first of the prophecies fulfilled, note how he responds:
Two truths are told,
As happy prologues to the swelling act
Of the imperial theme.
During his aside as he talks about his "horrible imaginings" as he thinks about killing Duncan to fulfill the final prophecy. Again, after this, Banquo describes him as "rapt." From his very first entrance in the play, Shakespeare seems to be stressing that Macbeth is a character who is consumed by an unhealthy sense of ambition that drives him to contemplate acts that are beyond the bounds of decency and respect. Even when, later on in the play, he seems to err on the side of caution and sees the danger of his ambition, his wife is easily able to sweep away his moral scruples and set him back on his ambitious course to perdition.
Key to consider is the way that ambition is something that seems to grow and drive Macbeth to commit ever more heinous crimes in the play. You might like to consider the way that he moves from killing Duncan himself to orchestrating the murder in secret of Banquo and Fleance and then how he openly slaughters all of Macduff's family.