Should I be scared or aware of something before I testify in court as a witness?

I witnessed something that involved a gun and stepped forward to the police. I could have been hurt; someone else could have as well. After the person was caught, which occurred literally seconds after I witnessed something, I came forward to the police.

Now I am being asked by the DA to testify in court because they are trying this person.

I’m nervous. I’ve never taken a stand before and I believe, whether this person deserves it or not, that it’s difficult to support putting someone in jail (I personally don’t think our correctional system is worth a damn). Should I be worried of who is there to support this person? I am worried someone will come after me, or threaten me (as what happened after I came forward to the police)... I guess I’m just looking for answers, knowledge, to prepare.

Am I even required by law to attend?