Okay, you say you're not at fault. However, some people (like the person who is suing you) want a judge to make that final determination, and they have the right to do that. Perhaps a judge might see things differently.
Anyway, if you had insurance at the time of the accident, it's time to give all of those papers to your insurance company so that they can defend you. It won't cost any money because a defense is included in the insurance premiums you are paying.
If you didn't have insurance, you may either :
1. Pay an attorney to defend you; or,
2. Buy a book on the subject from your local bookstore and attempt to defend yourself.