If your child is being bullied, or you think your child is bullying others, in the first instance, the best thing to do is speak to their form teacher at school. The school has a duty to make sure no harm comes to any child whilst in their care and should be able to suggest the best course of action.
If you're being bullied, there are many organisations that offer help and support. You can talk to specially trained counsellors about the problems you are having. They will offer you advice on what to do, who to tell and how to tell them. It is sometimes easier to talk to a stranger than a friend or your parents.
If the matter escalates and a criminal offence has been committed, then phone the police. Call 101 to report an incident that has happened, call 999 if the incident is an emergency and happening at that time.
Still not sure? Ask the Police.