how do i load a apple gift card to use on apps

An Apple Store gift card cannot be used for mobile device apps (iTUnes Store):

What type of gift card do I have? - https://support.apple.com/HT204199

Redeem App Store & iTunes Gift Cards, Apple Music Gift Cards, and content codes - https://support.apple.com/HT201209

Sorry, not sure what you are asking. If it is an Apple Gift card, and not an iTunes Gift card, then you cannot use it to purchase apps or media from the iTunes Store. If you are asking us how to redeem it:

Redeem App Store & iTunes Gift Cards, Apple Music Gift Cards, and content codes - Apple Support

Cheers,

GB