Want to activate my iTunes gift card

See the instructions in Redeem App Store & iTunes Gift Cards, Apple Music Gift Cards, and content codes - Apple Support

Open iTunes and scroll to the bottom of the screen.

Choose redeem.

You'll be asked for your Apple ID and password.

You'll be asked for the redeem code on the back of the iTunes card...you can either scan it with your camera or enter the digits manually.

Tap redeem. The amount will be added to your iTunes account.