Legal Question in Credit and Debt Law in California

You need to file an actual response to the complaint, usually a general denial (if the complaint is not verified) and affirmative defense. The first is a form, here: http://www.courts.ca.gov/forms.htm, the other you will need to research at a law library or online.

To get the proof that its valid, you will need to serve Interrogatories and Requests for Admission. And, possibly, Requests for Admission

Among the affirmative defenses could be that the debt is not yours, that the amount is overstated, and that the statute of limitation has run. Of course, these all depend on the circumstances.

You must file a responsive pleading (demurrer and/or motion to strike, or answer) to the complaint within 30 days of your having been served. If you don't do so, the creditor could obtain a default judgment and then try to collect from your assets.