Any lawsuit starts with a COMPLAINT, which states what the Plaintiff is asking for, typically an award of money. The defendant is also served with a SUMMONS, a document describing what will happen if the Defendant does nothing – generally the Defendant will be subject to having a DEFAULT JUDGMENT against him or her.  A JUDGMENT is a court order generally stating that the Plaintiff is entitled to collect a certain amount of money from the Defendant, and can use any means authorized by law to do so. 
Typically, a Defendant served with a Summons and Complaint has a certain amount of time (usually 30 days in California) to file an Answer, to prevent a Default Judgment. 

Everyone has a right to be their own attorney and represent themselves in a lawsuit – the first step of which is filing an Answer.
Here is a link to a form of an Answer that is generally acceptable for a collection lawsuit in California.
Once you prepare the answer, it needs to be SERVED on (meaning delivered or mailed to) the Plaintiff’s attorney.  You’ll need to get someone over age 18 who isn’t you to mail a copy to the Plaintiff’s attorney before you file it, then fill out this proof of service form.
Usually, you’ll have to pay the court a filing fee, called the First Appearance Fee, in order to have your answer filed with the court.  This ranges from $225 to $435, depending on how much you are being sued for.  If you can’t afford this, you might be able to get the court to waive the filing fees by filling out these documents.