You definitely want to call your county civil court ASAP. If you do not show up at court they will get a default judgement against you.
Here is a great website with lots of detailed info on the civil court process, including your possible Defenses and negotiating a settlement or payment arrangement in court without getting a judgement - IF they can prove the debt properly through the process of Discovery.
"http://www.nedap.org/hotline/court.html"
It is for New York and so may vary in some details with your state.
If you need an attorney but can't afford one, inquire with your court about Legal Aid. You can get an attorney at little to no cost to you.