Gift cards no credit card so how do I put money on Apple Pay

I do not have a credit card so I thought if I put a gift card on my Apple Pay I would be able to use it online but that is not the case so what I want to know is how do I put money onto my apple psy if I don’t have a credit card doesn’t make sense that you can’t use the money from gift card I thought that is what it was for if you don’t have a credit card you could use a gift card so I need to pay for something online now what do I do?