What would you do if you witnessed a horrible crime?

so earlier today, I was leaving cl for a dentist appt. and I saw this large black girl, named tonisha brown, she was stealing some kids bike that was chained to the bike ramp. She glared at me and now when I see her in the halls, her and her gang just stare at me. Then after lunch I found a fried chicken drumstick in my locker that had a note on it, it said I was "dead meat" if I told......what can I do, I am so scared.