How to write a letter back to the courts when being sued by a debt collection agency?

You will need to file an answer to the complaint with the court and serve it by mail on the opposing party within the twenty-one day deadline set forth in the summons.  The complaint is the lawsuit attached to the summons.  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, which denies the allegations in the 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 to the complaint.  File the answer to the complaint with the verification and an attached proof of service with the court and serve a copy by mail on the opposing party or the attorney for the opposing party.  The proof of service verifies the date of mailing.  You can either 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 eighteen 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 the proof of service 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.

If you don't timely file your answer to the complaint with the court and serve it by mail on the opposing party, the opposing party will get a default judgment against you.  A default judgment means you have lost the case.  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 then be back on track and litigation will continue.