You should speak to your child's teacher to start with to let them know of the situation and what your child has told you. It is important to remain calm but firm when approaching the subject.
Ask the teacher at school if they have noticed any changes in your child's behaviour, as bullied children are often withdrawn and quiet which can affect their overall school performance.
The school will have policy in place for dealing with bullying and the teachers will have experience in dealing with such, therefore they should be able to suggest the best course of action. The websites listed in Related Information offer help and support for parents and children about the best way to approach bullying.