A tattletale is a person who tries to get someone in trouble by revealing secret information about them. Your tattletale brother will probably tell your parents that you were actually at the movies, not the library.
The word tattletale is mostly used in the U.S. (in Britain it's more common to use telltale). It comes from the verb tattle, "report someone's wrongdoing." In the 16th century, you'd have called a tattletale a pickthank. These days, you can also use words like snitch or whistle-blower.