How to respond to a subpoena?

When I got home there was a subpeona left in my door stating I was being sued for a credit card debt that was opened in 2002. There is no court date set and only states that I have 20 days to respond. How do I do this? Do I respond to the court or to the company suing. This is an old credit card debt from a previous marriage. The company that now owns it accquied it in 2009 but I have never received and billings or paperwork from them regarding this debt. Any help would be greatly appreciated Thanks