Does anyone know what to do with a child around the age of 13 who suffers from serious anxiety?

Serious anxiety. Seek the consultation of a clinical psychologist. He or she may want to refer your child to a child psychiatrist for medication while therapy is ongoing.
Treatment. Find a psychologist specializing in children and a psychiatrist specializing in children. Also, take care of yourself as it is emotionally challenging to take care of a child with anxiety. Focus on your kid's strengths, to help create more positive experience for you and your child. Good luck.
Provider visist. Your child should be seen by your provider and possibly a therapist and psychiatrist. You need to understand why your child is so anxious and come up with a plan to help.