What to do if I received a letter from a law office for a credit card debt that I owe to a credit union?

Since you only received a letter, nothing will happen until you are served with a summons and complaint.  The complaint is the lawsuit attached to the summons.  When you are served with the summons and complaint, you will need to file an answer to the complaint with the court and serve a copy by mail on the opposing attorney within the time set forth in the summons.  The answer denies the allegations in the complaint.  If you don't file the answer to the complaint and serve it on the opposing attorney within the time set forth in the summons, the credit union will have a default judgment entered against you.  That means you have lost the case, but at that point you could still file a motion to set aside the default.  If the judge grants your motion to set aside the default, the case is then back on track and litigation will continue.

There won't be a wage garnishment until after there is a court judgment against you.  You can file bankruptcy to stop the wage garnishment.  It would be premature to file bankruptcy until there is a court judgment against you.  If you are eligible to file Chapter 7 bankruptcy, that is straight liquidation which would eliminate certain types of debts.  If you are not eligible to file Chapter 7, you can file Chapter 13 bankruptcy.  Chapter 13 requres a plan (budget) for repayment of creditors.

If you answer the letter you received, the law frim will probably want the entire sum now.  If you don't answer the letter, they won't file the lawsuit until the thirty day deadline is up.  Then after you are served with the lawsuit, you can file your answer to the complaint and proceed as mentioned above.  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 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 the answer 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 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 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.