The first thing you should do is — calm down, take a deep breath. It is going to be okay.
As long as you had liability insurance at the time of your accident, your insurance company will handle virtually everything having to do with the lawsuit.
The only exception might be if you have injuries of your own. In that case, the insurance company’s lawyer will probably file the initial paperwork to protect your legal rights, but you may want to hire an experienced plaintiff’s personal injury lawyer in order to present your claim for damages to the other side
I recorded a short video explaining what you should do if you unfortunately get sued: