The first thing to do after being served with a Summons and Complaint is to read the Summons. It will provide you directions on what to do next.
In general, a Summons requires the person served to answer the allegations in the Complaint in writing and file it with the Clerk of Court in which the lawsuit is pending and serve a copy on the opposing party within 20 days of service.
It is seldom a good idea to represent yourself and there are important defenses one can waive if not asserted in the Answer to the Complaint. It is best to see an attorney immediately for advice.