How do I get the balance on an Apple gift card I have?

You have to redeem it. Redeem your App Store & iTunes Gift Card - Apple Support

1. Peel or gently scratch off the label on the back of the gift card to reveal a 16-digit code that starts with X.
2. On your Mac, open the App Store.
3. Click your name or the sign-in button in the sidebar. 
4. Click "Redeem Gift Card."
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7. Click "Use Camera" and follow the instructions on the screen. If you have trouble redeeming the card, click "You can also enter your code manually," then follow the instructions on the screen. Make sure that you enter the 16-digit code that begins with X.

Can't redeem your gift card?

Using a Windows PC?

If you're using a PC, redeem your gift card in iTunes. From the menu bar at the top of the iTunes window, choose Account, then choose Redeem, then sign in and follow the instructions on the screen.