How do I use my Apple Gift Card to purchase something online

I was giving an Apple gift card for Christmas by my aunt I then uploaded it to the wallet on settings and it shows my balance of 25 dollars but I have another card that is expired and it's showing that I have no payment message right now I don't want to use it gift card as paper cuz it would take it all that stuff I'll be getting another debit card next week belong story short how do I make a purchase with that balance already in my account that's where I'm getting stuck at.