If I am being sued for credit card debt, can I file an answer or do I have to appear in court?

You will need to file an answer to the complaint with the court and serve it by mail on the opposing party within the time specified in the summons.  The time starts from the date you were served with the summons and complaint.  The complaint is the lawsuit attached to the summons.  If you don't timely file an answer to the complaint, a default judgment will be entered against you which means you have lost by default.  If that happens, you will need to file a motion to set aside the default.  If the court grants your motion to set aside the default, the case will be back on track and litigation will continue.

The answer to the complaint denies the allegations in the complaint.  At the law library, look for answer to complaint in the index of Pleading and Practice.  This will give you the general format for an answer to a complaint.  At the end of the answer is the verification which you sign and date under penalty of perjury.  The verification attests to the veracity of your statements in the answer.  File with the court, your answer with verification and attach a proof of service.  The proof of service verifies the date of mailing to the opposing party or opposing party's attorney.  You can use a court form proof of service or you can write your own.  If you write your own proof of service, it just says that you are over 18 and the attached documents were sent via first class mail unless stated otherwise to _________ (name and address of opposing party or opposing party's attorney) on _______ (date).  You sign and date at the bottom.  The date you sign should be the same as the date of mailing and the same date you file your documents with the court.