Gather evidence
Collect as much information as you can about what happened and who was involved. In many cases, your teen will know (or at least think they know) who is doing the bullying, even if it is in an anonymous environment or involving an unfamiliar screenname. Often the mistreatment is connected to something going on at school. If so, contact an administrator there with your concerns and make sure that an incident report and investigation is initiated in line with the requirements of school policy. Make screenshots or screen recordings of conversations, messages, pictures, videos and any other items which can serve as proof that your teen is being cyberbullied and submit them as evidence. Keep a record of all incidents to assist in the investigative process. Also, keep notes on relevant details like when and where the incident occurred (at school, on specific apps), as well as who was involved (as aggressors or witnesses).