Usually personal service is the method for serving a summons and complaint.  It is possible that you could be served by notice and acknowledgment of receipt which means that the summons and complaint were sent by mail with a form for you to sign and return within a period of time stated on the documents.You will need a case number in order to file your answer to the complaint with the court.  The complaint could not have been filed with the court without a case number.  It is possible that the copy you received inadvertently omitted the case number.  If the complaint was filed with the court, the court clerk would have stamped it with a case number.  In order to find out if there is a case number, look in the plaintiff/defendant index in the court's computer.  Look for your name under defendants.  If the case is listed in that index, a case number will be listed.Your answer to the complaint denies the allegations in the complaint.  At the law library, look in Pleading and Practice.  Look in the index under Answer to Complaint.  This will give you the general format for an answer to a complaint and examples of answers.At the end of the answer is the verification which you sign under penalty of perjury. The verification attests to the veracity of your statements in the answer to the complaint.  File your answer with the verification and an attached proof of service with the court.  The proof of service verifies the date of mailing to the opposing party.  You can either use a court form proof of service or write your own.  If you write your own proof of service, it would just say that you are over 18 and the attached documents were sent via first class mail unless specified otherwise to _______ (name and address of opposing party) on ________ (date).  You sign the proof of service under penalty of perjury and date it.  The date you sign should be the same as the date of mailing and the date you file the answer with the court.If the plaintiff can't prove you owe the amount, the plaintiff won't be able to get a judgment against you.Make extra copies of your answer to the complaint, verification and proof of service so that you can get a conformed (court stamped) copy for your records when you file your documents with the court.If you don't file an answer to the complaint within the time stated on the summons, you will lose by default.  Then, you will need to file a motion to have the default set aside.