The first thing you need to do is file an Answer which is a written response to the lawsuit. You will need to send that to the creditor and file it with the court. After you file your Answer, you should receive a court date in the mail, make sure you go to court. Failure to attend or file an Answer can result in a default judgment against you. This means the bill collector will get everything they asked for in their Complaint. Also, consider filing bankruptcy. If you can file bankruptcy, your bankruptcy attorney can file an Administrative Stay with the court which will prohibit collection efforts against you. If the lawsuit is based on an unsecured debt (ie. credit card, certain utility bills, medical bill), you may be able to get the entire debt discharged through the bankruptcy case. Good.
Don't panic and don't lose hope. First of all you may qualify for legal aid ... get in touch with the local bar association and the law schools that are nearby to see if local attorneys are helping poor people on a pro-bono basis.
The age of the debt may bar suit on it ... you need to know the statute of limitations in your state and whether you defaulted on the debt so long ago that the statute is triggered and should be plead by you.
Also if the party suing on the debt is not the original creditor (a "junk debt buyer") they often lack the documentation necessary to make the case valid in court, and can be defeated on that basis.
With no income or assets, even if you lose the case the judgment is "wallpaper" for them. And if you move out of state they would have additional legal hurdles and practical problems collecting on it in your new state.