This might be some sort of username and password or certificate depending on the service, and it will vary from provider to provider. Your VPN provider should also provide manual sign-in information that can be used as credentials. It will differ depending on the VPN service you're using, but you should have access to a webpage or desktop app section that lists the server's location, the address needed to create a manual connection, and the current load of the server. Once you subscribe to a VPN service - keep in mind there are plenty of great VPN options out there - you should have access to a list of servers that you can connect to.
