If I just got served with a subpoena on a first degree murder case and I do not want to testify for fear of retaliation. is there anything that I can do?

If you are afraid of some retaliation from the defense with respect to the trial subpoena you were served with presumably from the district attorney's office in this first degree murder case, you need to consult with the district attorney's office about your concerns and how to possibly get some witness protection.