If you have a copy of the summons in hand, you likely should read the summons carefully as it likely informs you what you must do to answer the summons. There likely were other documents served along with the summons. You should read those documents carefully. Depending on why you are being sued, among those documents may be notices of an upcoming hearing.
Usually, the person being sued (called the defendant, respondent, or something similar) has only a short time to respond. If the defendant does not respond with the deadline, the plaintiff generally may ask the court for an order of default.
Most people have never been a party in a lawsuit and likely never will in their lifetime. If you, like many people, do not know what to do, you may want to consult an attorney.
Are you the named defendant/respondent to the civil action?
NO, I AM NOT A DEFENDANT/RESPONDENT:
In Washington, a civil action is commenced by (1) service of summons and complaint, or (2) by filing a complaint (normally, the moving party will do both simultaneously). Assuming the summons was properly served (another legal issue), you must appear. Notify the court ASAP if you cannot appear.
YES, I AM A DEFENDANT/RESPONDENT
The summons should have come with a complaint. The complaint must include the jurisdiction and venue, a factual statement, right to relief, and the relief requested. You must answer the complaint. The answer is normally due within 20 days after service of the summons/complaint. The answer may include a general denial of the allegations, affirmative defenses, etc. Your answer may also move for dismissal for lack of jurisdiction, insufficiency of service, failure to join a party, or failure to state a claim upon which relief can be granted.
If you do not respond to the summons/complaint (fail to appear or answer), the moving party (plaintiff) may present proof to the court that you failed to appear. They may also make a motion for default judgment, which may be enetered immediately or any time after the entry or the court's finding that a party is in default. If the amount of the judgment is for a known, specific amount, the court may award such amount. If the amount is uncertain, the court may order hearings or other procedures to determine the proper amount of damages.
In short, give your best efforts to honor the summons.
Florida law usually gives you 20 calendar days to file an Answer to the Complaint for certain civil actions such as Foreclosures. More time is given for complex civil actions. However, if you are unable to file an Answer in court within that 20-day period, you can at least file a Notice of Appearance of ask an attorney to file a Notice of Appearance, and then file a Motion for Extension of Time to File an Answer.
Be careful about filing an Answer to the Complaint or an Answer to the civil suit summons right away. I don't know about other states, but in Florida, filing an Answer or other responsive pleading waives any grounds for dismissal. This is why you should get an attorney to look at your case. If the attorney finds that there is at least one ground for dismissal, he or she may successfully get the case against you dismissed. This will force the other party to come to the negotiating table or start all over again, buying you more time. If your Answer to the Complaint is likely to be complicated or lengthy, or if you are in a particularly bad situation and have little or no valid legal defense, it may be helpful if the attorney buys you some time.
With that being said, if you hire a good attorney, he or she may be able to insert some strong legal defenses and even counter-claims in your Answer. It all depends on the facts and circumstances of your case, nothing is set in stone. But just the fact that you have been summoned for a civil suit indicates that you at least need to show up and show the court your respect. It goes a long way toward helping your case.
Avoid getting a default judgment against you. The court and the clerk of courts need to know that you exist, that you are paying attention, and that you really care about the outcome of your case. Otherwise it's very easy for the other party to get a default judgment against you, and for them that's like winning a title fight while the referee held your hands behind your back. Good luck on your case.