Felix: Lucy, the party is tonight and we have a few more things to prepare.
Lucy: Just let me know how I can help and we'll get it done together.
Felix: Balloons. We needs lots of blue balloons. Can you go to the party store and take care of that?
Lucy: I'm on it!
Mom: Richard, have you prepared your college applications yet? They are due in a week.
Richard (playing video games): Don't worry, mom. I'm on it.
Mom: Well I hope so. This is your future at stake!
to be on it is an inseparable phrasal verb. Change the be verb to match your tense and subject. This expression is usually used in the affirmative, rarely in the negative form.
I am on it.
You are on it.
We are on it.
He/She is on it.
You can use this expression when a person is working on a task or project and it is under control. In example 1, Lucy takes on the task of getting balloons. She will take care of it. In example 2, Richard assures his mom that he is taking care of his applications. This expression is used when there is confidence that the task will be completed.
I'm on it = I will do it = It will get done