If I received summons/complaint for unpaid medical bill, where doI go from here?

You will need to file an Answer to the Complaint.  If you don't file the Answer with the court in the time set forth in the Summons for a response, you will lose by default.

The Answer denies the allegations in the Complaint.  At the law library, look in Pleading and Practice.  Look in the index of Pleading and Practice under Answer to Complaint.  This will give you the general format for an Answer and sample Answers.  At the end of the Answer is the Verification which attests to the veracity of the statements in your Answer.  The Verification is signed under penalty of perjury.  Include the Verification at the end of your Answer.  File your Answer to the Complaint with the Verification and an attached  Proof of Service with the court.  The Proof of Service verifies the date of mailing to the opposing party's attorney.  You can either write your own Proof of Service or use a court form.  If you write your own Proof of Service, it just says that you are over 18 and the attached documents were sent via first class mail unless specified otherwise to  ____________ (name and address of opposing attorney) on _________ (date).  Sign the Proof of Service at the bottom under penalty of perjury.  The date you sign the Proof of Service should be the same date as the date of mailing stated in the document and the same date you file your papers with the court.