How can I help my child to overcome her shyness?

Make her feel safe. Kids are normally shy when they are young. You do not mention your child´s age, but being shy is part of a personality trait and not a disease. Most people, adults and children alike feel shy" when they around strangers or large gatherings, not to mention when they have to give a presentation at school. Let her know it is ok to be shy and avoid pushing her. She will blossom when it is right.