If a creditcard company filed a lawsuit against me, what should I do?

You will need to file an answer to the complaint with the court and serve it by mail on the credit card company or its attorney within 20 days from the date you received it.  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.  The answer 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.  File your answer to the complaint with the court with an attached proof of service.  The proof of service verifies the date of mailing to the opposing party.  You can either use a court form proof of service or you can write your own.  The proof of service just says that you are over 18 and the attached documents were sent via first class mail unless otherwise specified to __________ (name and address of opposing party) on ________ (date).  The proof of service should be signed under penalty of perjury and dated at the bottom.  The date it is signed should be the same as the date of mailing and the same date it is filed with the court.

If you don't file your answer to the complaint within the 20 days from the date you received it, the opposing party will file a default.  You will then lose 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.