It's called a "Notice of Intent to Defend" or "Notice of Appearance" and you usually have to have it returned to the courthouse by the date on the summons. A form for that may be attached to the summons, or you may send in a separate one, or do both (I did both). As ScottGem said, search the forums for examples and procedures, as this is a common question posted here. In fact, many of your questions in general, have most likely already been answered. I found many answers to mine, and really didn't have to post much regarding specifics.
When filling this notice, you need to answer their claims with defenses. Examples of "answers" have been posted by mr yet and others. In addition, you can also provide "affirmative defenses" . Answers and notices should be sent to court and plaintiff and copies retained by you, the defendant. Send certified mail, return receipt requested. A "Certificate of Service" should be sent to plaintiff (notarized), and a copy sent to court clerk (check court rules to see if it needs to be notarized, most don't), and make copy of the one you notarized and sent to plaintiff, for your own records. Your state should have a website for the court, where you can find specific court rules, plus forms you might need. You can also ask the court clerk for assistance. You also should look up your state's rules for court procedures, and for debt collections.
Depending on specifics, there may be other things you should do. Such as file an exemption for income and assets that are exempt from garnishment/seizure, if you have anything that could be taken in event of a judgment from plaintiff. And if you haven't already, you can request validation of the debt, if you question the amount, or whether it's yours, or whether JDB is the proper entity to collect. If you acknowledge it's yours, you may want to work out a settlement with the collection agency. Each of these different steps have been spelled out in previous threads.
If you are planning to represent yourself as Pro Se Litigant in a civil hearing, it is imperative that you take the time to read the forums, including the sticky threads at the top. Read about others cases, what you should, and should not, do. Read all the federal and state laws regarding debt collections and court proceedings. If you go into this, without a great deal of preparation, you are not going to succeed. Remember: Knowledge is power. Once you have found answers to general questions, forum members can assist with specifics. All of the terms I put in quotations ("-"), can be c&p'd into the forum search box for examples, procedures and templates. If this seems too overwhelming, or the amount is a considerably large sum, consider hiring an attorney. Forum members cannot give legal advice, per se, but they can guide you into making legal decisions for yourself, and assist you on proper legal procedures.