Nominative is the subject of the sentence (i.e. the person or thing which does the action) and accusative is the object, i.e. the person or thing which the action happens to. So, for example:
I ate poison
You ate poison
You and I ate poison
Poison killed me
Poison killed you
Poison killed you and me
Incorrect examples:
Me ate poison
Poison killed I
You and me ate poison
Poison killed you and I
More examples:
He ate poison
She ate poison
He and she ate poison
Poison killed him
Poison killed her
Poison killed him and her
Wrong examples:
Him and her ate poison
Poison killed he and she
Words like I, he, she, we, they are nominative and words like me, him, her, us, them are accusative. Words like you and it are the same in both.