Many people FREAK OUT when they get served with a credit card lawsuit. Don't. It is a CIVIL lawsuit, which means you are not going to jail, and you are not in trouble in any criminal context. Calm down, realize that you are not in trouble, and that everything is going to be alright.
Decide what to do
You have two good options: Fight the lawsuit or file Bankruptcy. The third, bad option is to do nothing. That is really dumb. If you do nothing, they will get a judgment against you for sure. If you cannot pay this lawsuit, have other debt problems, etc., call a Bankruptcy lawyer and see if you should file a Bankruptcy. It would get rid of the lawsuit and clear your other debts as well. If you cannot file bankruptcy, you should still answer the lawsuit. 90% of the time, if you file a simple answer, you will win the lawsuit (see below).
File an Answer
If you ignore this lawsuit, they will get a default judgment which gives them permission to take money from your bank accounts, put a lien on your house, mess up your credit, and make your life hell. So you must answer it. You have nothing to lose. Prepare a one page Answer that has your case number, name of the case (Credit Card vs. John Smith). Type: "I deny all allegations in Plaintiff's Complaint." Sign it. Go to the court where your were sued, find the clerks office, and tell them you want to file your answer.
Wait to hear something
The court may set a trial date, which is fine. It is likely that before the trial date, the credit card company will dismiss your case, file a non-suit (cancel the case), or back down some other way. Sometimes they do this the day before the trial day. If you end up having to go to trial, and you do not have an attorney, go and tell the judge that they do not have any evidence that proves the amount of debt (this is usually true). Even if it gets to this point and you lose, you will probably fare better than if you laid down and let them get a default judgment against you.