What is needed to prove destruction of property?
What the defendant (that is, you) says to other people is admissible in court; it is not hearsay. (That is why for example, you read about people in jail who get convicted because they admitted what they did to their cell mate.) If the person who claims you told him that you would break the property is credible (believable), and you had means, motive and opportunity, they could prosecute and potentially convict you based on that.