Don't fill out forms without first consulting a lawyer, preferably I'm person.

You will want to file a responsive pleading to the complaint within 30 days of having been personally served with the summons and complaint. There are different types of responsive pleadings: 1) demurrer, 2) motion to strike, and 3) answer. (In some cases, a defendant might want to take the case to federal court.) Most people will file an answer with the court and mail a copy to the lawyer representing the plaintiff, i.e. the person or company who filed the lawsuit. Some lawsuits, though, might not be legally sufficient to apprise the defendant of the matters being alleged. In that case a demurrer would be appropriate instead of an answer. Similarly, if there is matter in the complaint that is irrelevant or not legally proper, a motion to strike would be appropriate. Sometimes, defendants would file both a demurrer and motion to strike at the same time, after getting a hearing date from the court, and paying the appropriate filing fee.

The Judicial Council website has forms for answering the complaint. But If this doesn't make sense to you, I agree with the first part of Mr. Stone's response, that it would be best to obtain advice from a licensed attorney.

The reason you don't want to fill out some form is 1) you may be giving up important rights if you fill out the form incorrectly; 2) contrary to the totally false language on the summons form, it takes a lot more than filling out a form to defend a lawsuit; and 3) many types of lawsuits provide for an award of attorney fees if you win, so why wouldn't you at least consult an attorney?

To answer your question directly, there are forms for responding to the complaint. It is a response to the complaint that is required, not a separate response to the summons. You should have been served with a copy of the complaint when the summons was served. You were probably also served with other documents, including a civil case cover sheet, a civil case cover sheet addendum, notice of ADR, etc.

The California Courts' website has forms for an answer that is fillable. You can find that website here: http://www.courts.ca.gov/forms.htm

With that said, I agree with the other attorneys that you should seek legal advice immediately before filling out any forms and setting off on your own.