Google is your "fat man" ...
What you're looking for isn't just a simple upload script, you need to have
a complete user register/login system with admin file upload, a credit card
payment gateway, and a download system for the people who payed.
I'm showing you PHP script examples.
Some you have to pay for, others might be free.
Some have minimal features, some have too many features.
And the language is relevant because your choice may determine different kinds of servers.
I'm also guessing you'll have to hire a programmer or someone to help you with this.
Because you're not sure what to search for on Google, I can only assume you are not a programmer.