How to go about a wrongfully issued summons?

First, contact the plaintiff (person suing) or, if he/she has a lawyer, the attorney: explain what you just explained to us, and see what documentation they want (if any) and you can provide to show this was not you. This could be an innocent error (e.g. the real defendant may have the same or a similar name, so when they searched for the defendant, they came up with you), and if you convince the plaintiff or lawyer that you're not the right person, they can voluntarily dismiss the case. (If they agree to do this, ask to be copied on the letter of dismissal; then call the court to make sure it occured).
If they won't agree to voluntarily dismiss the case, you will have to file an answer, if the summons says that you must; in your answer, deny all the incorrect facts and also affirmatively state that you are not this person, did not own a car then, had not been in an accident, etc. Then make a motion to dismiss based on you not being the correct defendant. You should be able to get instructions and possibly sample or template forms from the court for the answer and the motion. If you don't answer, then right defendant or wrong, you will lose by default.
If the summons does not require an answer (e.g. it is for small claims court, and in many small claims court, there is no answer) and the other side does not voluntarily dismiss, you will need to show up in court and make your case to the judge.
Obviously, if they won't voluntarily dismiss, an attorney would be helpful, but you do have the right to answer, appear in court, etc. yourself.