to rat (someone) out means that you are going to tell an authority (teacher, parent, police, etc.) that someone broke the rules, laws, etc.
Don't rat me out to the teacher for stealing Sarah's pencils, please!
He is going to rat James out to the cops for breaking into the neighbor's house.
They want to rat him out for ripping pages out of library books.
Usually, we don't say "I'm going to rat (someone) out" because it has strong negative meaning. When referring to ourselves we usually use "to tell on"
I'm going to tell on you for ripping my shirt.
She told on him for breaking her toys.
I hope that helps ;)