File a document at the courthouse called "AN ANSWER". It lets the judge know that you want to be involved in the lawsuit.
If you do nothing, a default judgment can be taken against you.
I sell ANSWERS for $25.
They are free to file at the courthouse. You need to have at least 3 copies -- the original goes to the court, you keep a copy and you send a copy to the opposing attorney (the one who served you!).