How to create a login system online?

Well, I wanted to create a login system for my game to store up a database online, logins and passwords, if the person had not signed could only access the demo of the game that would be installed on the user's computer and just use the Internet to obtain the login and password, to have a login was required if you paid me (I have the Unity Pro), I was paying I would create a validation script to validate the login and released the full access to the game, I would know how to do it all or if you have any tutorial that could help me make the system, I saw something with phpMyAdmin, but I know nothing about it, I wanted your help with a tutorial or explanation of how to get the system !

Thank you for everything!