If you are insured, you should contact your insurance company immediately. Most automobile liability policies provide for the company to provide you with a legal defense in the event that you sued in conjunction with an automobile accident.
Even if you believe the other driver does not "have a case," you should not be unrepresented. You will need someone to respond to her complaint.
The above information is shared for educational and discussion purposes only. No attorney-client relationship is intended or established through your reliance on the information provided. If your legal rights could be impacted by using this information, you are urged to seek legal counsel before taking action.
You need to let your auto insurer know about this ASAP. They will provide a defense for you. If what you are saying is true, then it sounds like she should lose. Depositions of all witnesses will be necessary, including the drivers. If your insurance company sides with you, then they will fight this. If they think the other side will win, they will attempt to settle for an amount somewhere within your policy limits. The fact you are being sued tells me that the other driver has already attempted to settle with your insurance company, and they denied the claim or made a very small nuisance value offer. Either way, no need to worry as long as you had auto insurance.
No need to get a lawyer if you had auto insurance at the time of the accident. Let your insurance company know about the lawsuit ASAP. They will assign an attorney to defend you free of charge. If you did not have auto insurance at the time, then you will definitely have to hire an attorney to defend you.