Get a lawyer! If you're in fear of your wellbeing, you have certain rights, and a good lawyer from your state can help get you out of it safely, and without being in trouble with the law. You must respond to the subpoena though, or as Nangiggles says, you can be charged with contempt and be arrested. If you aren't in a position to get a lawyer, call the DA (or to the defendant's lawyer; whichever got the subpoena), and go talk with them before the court date, so you can find out what they want you to say, and try to make a deal with them yourself.
Charles