You can respond in writing to the Complaint with an answer. There is a strict time limit, though, so you must submit your answer before the deadline.
You can then appear at a hearing and explain your side of the story. The creditor must prove that you actually owe the debt. Often the creditor cannot produce the necessary documents to show that you actually owe the debt. Even if the creditor can produce the documents you may be able to prevent the creditor from entering the documents into evidence. At this point, it may be helpful to hire an attorney to argue your case at the hearing since it can get complicated.