Unauthorized access to Apple gift card within 24 hours

This afternoon I bought a apple gift card. The reciept says that is acrivated but it will only be fully activated within 24h.

Someone had unauthorized access to the 16 pin number.

How do I check if it was used?How can I cancel it, once its with th 24h. I jave the card and the pirchase reciept. Its my first time using apple products and I dont know my way arround.

Thank you for your help.

[Re-Titled by Moderator]