NEED ADVICE REGARDING BEING A WITNESS OF A CRIME

FROM MASSACHUSETTS: I HAD MY CAR STOLEN RECENTLY, WAS ACTUALLY CAR JACKED BY A PERSON I PICKED UP WHO WAS SUPPOSE TO SHOW ME HOW TO GET TO A MALL IN HIS CITY WHERE I WAS LOST. HE INSTEAD STOLE MY CAR. HE HAS BEEN CAUGHT AND ARRESTED, NOW DISTRICT ATTORNEYS OFFICE SENT ME LETTER THAT I MAY BE CALLED AS A WITNESS AT HIS TRIAL. HE STOLE MY REGISTRATION AND HAD MY KEYS TO CAR AND HOME SO HE KNOWS WHERE I LIVE. I WOULD RATHER NOT BE INVOLVED IN THE PROSECUTION AT ALL IN ANYWAY OUT OF FEAR. I SIMPLY NEED TO KNOW UNDER THE LAW WHAT MY OPTIONS ARE. CAN THE STATE JUST PROSECUTE HIM ON THEIR OWN SINCE HE WAS CAUGHT IN THE CAR BY POLICE AND ARRESTED. DO I HAVE TO APPEAR IN COURT OR CAN I SAY I WANT NO INVOLVEMENT?
THANK YOU FOR ANY INFORMATION YOU CAN GIVE ME

I am a law school graduate. What I offer is mere information, not to be construed as forming an attorney client relationship.

Well, they need you to substantiate it was stolen and how he got the car (i.e. that you didn't just out of the goodness of your heart give to him like in the movie pay it forward). Also, if they certain aggravated charges on him, new car jacking laws are very serious as well, they need you to illustrate to the court the violent circumstances.