First bit of advice: Find a lawyer.
Did they actually serve you? Or are they just threatening to sue? If they are threatening to sue they could be in violation of the Fair Debt Collection act and you can sue them for $1000 for each time they do this.
If they served you, (first get a lawyer) and go to your courthouse and file an Answer to the Complaint. For affirmative defenses, challenge whether they own the debt and challenge the amount of debt. Demand that full statements and signed agreements be provided and a schedule of all fees that add up to the full amount of what they are suing you for. I this is a charged-off debt and they bought it from someone else, this will probably stop them dead in their tracks. If they are the original creditor you will be notified of a court date some time in the future. You can also request mediation to try to come up with a payment arrangement that you and your creditor can both live with.
Hope this helps, but laws are different in every state, so ask a lawyer about this first. Hopefully you can find a friend or family member who can give you some free legal advice or maybe even represent you. Good luck!