First you should remain calm and do not get irate or offended with the individual who is making the false accusations. Provide some factual information to prove that the accusations are wrong, and also name and use credible witnesses that are able to speak on your behalf regarding the situation. Find more information here http://www.ehow.com/how_2168917_handle-being-wrongfully-accused.html.
If you did not do what you are accused of then stand your ground and stand up for yourself. Depending on what it is their should be a way to prove your innocence. If it's a legal matter and you might get in trouble then you need to get a lawyer. Don't get railroaded into taking the blame for something you didn't do. GL