What can I do when I received a subpoena from a law office to collect a debt. What do I need to prepare for the court date?

There are several defenses that you can argue in court.  For one, the debt itself.  As you rightfully pointed out, your creditor needs to validate it. 

If you believe that the amount of the debt is incorrect, you have the right to dispute it.  The plaintiff must prove that the principal, interest, collection costs, and related fees are not only correct but also agreed to in your contract and properly charged.  In fact, you have the right to your see original contract, account statements, and purchase receipts.

Again, there are several other defenses that you may want to try and assert.  I'm providing you with a link to a site that will explain further.  It is for NY courts but the general defenses are the same (certain time requirements and other similar factors will differ) and it will give you a good idea as to how to proceed in court:  http://www.nedap.org/hotline/defenses.html