Keep in mind that when you do this assignment, you're dealing with Macbeth as Shakespeare described him in the play, not necessarily the historical Macbeth (at least I'm assuming that's what your teacher wants). The real Macbeth wasn't the horrible dictator that Shakespeare painted him as (Shakespeare was writing to please King James I, who was supposed to be a descendent of Banquo), so you might want to stress in your assignment that you're talking about the literary Macbeth, not the historical Macbeth.
Comparing the literary Macbeth to Adolf Hitler is quite interesting, but you're going to need some background information on Hitler to back up the point of your assignment (your thesis, we might call it). Because we don't know anything about Macbeth's early life, you may want to just compare the rise to power of both men. For example, did Hitler have to kill someone already in power to become Chancellor of Germany? No, his popularity in a time of national recession led to his election, and his powerful persuasive speaking. So if their rise to power wasn't similar, what was similar about these two figures?
Both of them exhibited some serious signs of mental instability. Macbeth's instability began when his beloved wife all but pushed him into Duncan's chamber to stab him to death, and you can find quotes in the link below pertaining to madness in Macbeth. Hitler's instability and obsession with creating a pure Aryan race is well-documented, too. Good luck!