How can I get refund for fake Apple Gift Cards?

Please follow the steps in this Apple Support article. 

If you can't redeem your Apple Gift Card or App Store & iTunes Gift Card - Apple Support

If after reviewing the article, and following the directions in the article, you need additional assistance please follow the link below to contact official Apple Support. 

Contact Apple Gift Card Support

If you receive a request for payment using Apple Cash, or If you believe you are the victim of a scam, please contact Apple Support and report it to local law enforcement.

About Gift Card Scams - Apple Support

You may also report the scam to the FTC Complaint Assistant and to the FBI’s Internet Crime Complaint Center

If you receive a request for payment using Apple Cash, or If you believe you are the victim of a scam, please contact Apple Support and report it to local law enforcement.

You cannot. You'll need to try the merchant or person from whom you purchased the fake cards. Most likely, you are out the money.