Learners should avoid this rude word like the plague. Nevertheless learners may see or hear it, and it has a wide range of possible meanings in AmE.
"You are sh*t", "You ain't sh*t" and "You are a sh*t" are all insults.
But "You are not sh*t" is a reassurance.
And "You are not the sh*t" is another insult while "You are the sh*t" is a compliment.
Things like this are one reason English is hard to learn.

Retired English Teacher