Received subpoena to testify against someone I don't know.

Yes, you have to comply with a subpoena and show up. You can certainly contact the attorney who caused it to be issued, explain that you think it was a mistake and that you have nothing to testify or say...however, if they do not retract the subpoena, you will need to testify. A failure to obey a subpoena could make you subject to penalties.

Yes, you still have to show up.  If called, you take the stand, and you answer every question put to you as honestly as you can.  Not showing up is contempt of court, lying is perjury, and either one can land you in jail.

If you believe that there is even the slightest chance that you are under any suspicion yourself, by law enforcement, please talk to a criminal defense lawyer before you go to court;  it's best to do this as soon as possible, because sometimes, on the right facts, a subpoena can be quashed, but that takes a motion ahead of time.