You will have to answer when you get served. You will need to research your defenses...get started on that asap. You can DV them through discovery...you will need to find out what the rules of your particular courts civil proceedure is. don't worry, there are tons of GREAT people on this board who really, really help....
What state are you in?
Which Atty is suing?
Who is the Original creditor?
How old is the debt?
Last time you paid on it?
Someone will be along shortly that is experienced...I wish you the best of luck on your case!!!