Use "are not" when the subject is "you" or any plural.
You are not good.
We are not good.
They are not good.
Dogs are not good.
John and Mary are not good.
Use "is not" when the subject is singular, but is not "you" or
"I."
He is not good.
She is not good.
It is not good.
John is not good.
Mary is not good.
Use "am not" when the subject is "I."
I am not good.