What do I need to do if I just received a summins for a credit card debt?

You will need to file with the court an answer to the complaint (the complaint is the lawsuit attached to the summons) and serve a copy of your answer to the complaint on the opposing attorney.  You will need to file your answer with the court and serve it on the opposing attorney within the time set forth in the summons.  If you don't file with the court and serve the answer on the opposing attorney within the time set forth in the summons, the opposing attorney will obtain a default judgment against you.  A default judgment means you have lost the case; however, if that happens, you can still file a motion to set aside the default.  If the judge grants your motion, the case will then be back on track and litigation will continue.

As for your answer to 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.  The answer to the complaint 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 your answer to the complaint with verification and an attached proof of service with the court and mail a copy to the opposing attorney.  The proof of service verifies the date of mailing.  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 eighteen and the attached documents were sent via first class mail unless stated otherwise to ___________ (name and address of opposing 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.