The entire play is important, but I agree that you should be able to see the metamorphosis of Macbeth's character from noble and respected warrior to butcher of women and children. Read all of Act I, Act II scenes 1 and 2, Act III scene 1, and Act V scene 1. These parts represent Macbeth's downfall and the queen's part in all of this. Good Luck!
You should start in Act I, scene v, where Lady Macbeth is reading a letter from her husband, announcing that a prophecy was made that he may one day be king. We start to see her ambition at any cost personality coming through in her response to this letter.
LADY MACBETH: ...thou wouldst be great;
Art not without ambition, but without
The illness should attend it:... (1. 5)
Continue through the rest of Act I, which shows her excitement that King Duncan is coming to stay at their home, thus giving Macbeth the perfect opportunity to kill Duncan and become king.
After that, you can see how she continues to push and push her husband and herself onto the throne of Scotland, making her a "butcher and fiendish queen" indeed!
Malcolm: ...this dead butcher and his fiend-like queen,
Who, as 'tis thought, by self and violent hands
Took off her life (5.8)
However, to give a well-rounded character sketch of Lady Macbeth, one must also read through to her sad end in madness and despair over her guilt.