One option is to simply not respond to the lawsuit. If you fail to respond within the specified amount of time, you will lose your case and your chance to defend yourself in court. The plaintiff can then file for a Request for Default. Once the default is entered, the plaintiff can start collection actions against you, such as garnishing your wages or levying your bank account.
Some individuals make the decision to let the case go to default, particularly if the filing fees and other costs associated with defending a lawsuit are more than what is being asked for in the complaint, and the defendant has no defense to the claims. In some cases, if the plaintiff wins the case in court, attorneys' fees and court costs can be added to the judgment, meaning you could end up owing more if you defend the case and lose than if you let the case simply default. However, this can be a complicated and difficult determination, and you should carefully consider your options, or consult an attorney for the best advice.
Also note that once a default judgment is entered, under certain circumstances you can ask the court to cancel, or "set aside" the default.