can u please tell me
when we can use
you are on it
i am on it
The expression "I am on it," aside from its literal meaning, can mean, "I will take care of this."
Boss: "We'll need to call all the North American offices and tell them delivery will be delayed."
Employee #1 (picking up phone): "Don't worry, I'm on it."
Boss: "And what about the overseas offices?"
Employee #2: "I already told Sandra to put calls through to all the European offices as soon as they open for business."
Boss: "Oh, you're already on that. Good."
Is this the meaning you are looking for?
[native speaker & writer but not a teacher]