English

Betty: Fred, I found out about you and that other girl. How could you do this to me?!
Meaning: if you find out about something, you discover a fact or information that you did not know before, often because someone was keeping it secret.

Meaning: if you tell on somebody, you give information about someone, usually about something bad that they have said or done, to a person in authority (like a teacher, boss or parent), knowing that this will probably result in punishment.
Example sentence: When I was a child I used to steal apples from the tree in the neighbour's garden but my brother told on me and my father grounded me for a month.