I received a summons AFTER a charged off credit card had been paid in full. How do I respond? Never been sued before.

IN all likelihood the summons and complaint was in the works long before you paid your debt and you should be able to get the action withdrawn.  You should call and write the attorney representing the plaintiff and provide him/her with proof that you  paid the debt.  If he/she doesn't voluntarily withdraw the claim by the time your answer to the complaint is due, respond to the complaint.  Depending on what court you're in (different courts have different rules) you can respond either by moving to dismiss the case based on documentary evidence, e.g. your documentary proof of payment, or simply file an answer denying that you owe the money because you have already paid it. If you can't afford or simply don't want to hire an attorney, speak with the clerk of the court for help with the procedure.