To tell is to describe or announce something, either by speaking or writing. If you're going to be late to a movie, you should tell your friends so they can save you a seat.
You might tell someone to be quiet in the library, or tell a bedtime story to your siblings. You can also tell your friend a secret or tell her secret to someone else. Informally, to tell on someone is to inform an authority about something that person did that she wasn't supposed to do. And there's also a way to use tell to mean "determine with certainty," like when you can tell it's going to rain because the sky is so dark.