Create GameKeys Shop?

I thought I would like to create a GameKeys shop such as Xbox cards Psn cards Steam cards etc. And wanted to ask how I do it

I think if you don't even know how to do something like that and have to ask here, that's probably not for you. Are you of legal age at all?

Yeah, but I'm still playing Fortnite

That wouldn't be for someone who obviously has no experience with such things. You'd have to put some money into your hand anyway to set something up and even then it will most likely not generate any profit at first.