Storage on an iPhone

Assuming that you mean an Apple Gift card, redeem it according to these instructions.

How to redeem your Apple Gift Card or App Store & iTunes gift card - Apple Support

If you have money in your Apple ID balance from gift cards, your credit or debit card will not be used to pay for the storage. The money in your Apple ID account will always be used first. I pay for my Family iCloud storage with gift cards.

Be aware that you can't buy more storage for your phone, it is a fixed size. You can purchase a monthly subscription for more iCloud storage.