On initial setup automatically I already have "ROOT" user, but I read on tutorials here on Digital Ocean I need to create new superuser or user with same privilage as root. Why I have to do that if it has exactly the same privilage?
The root user is a "dangerous" user, since it can do everything on the system. So to protect yourself, it's highly recommended to create a user that has sudo capabilities, since it will require that another step to "accidentally" doing something at the system level. It also adds another security layer.
If you're familiar with Windows, then using root would be like running as Administrator, which is also a security concern, so on Windows it's recommended to create a non-Administrator user and specifically run things as Administrator when needed.
I think I get the gist, I also just Googled it too, pretty neat, you're right about it's like to run as Administrator on Windows, and also it seems like Windows Group Policy too. But I still don't know when I should use it if I'm the only person who's going to use this VPS? Should I login as superuser once I create it?
Another question, for superuser I have to add "sudo" at the beginning of the command to run root command while I don't have to when I'm using root user? Sorry for the obvious question for you, I'm really new with this :D
If you were setting up Windows for someone who had never used it before - or kids - would you then setup their user as Administrator?
If your answer is yes (which I hope not), then just use root as the only user.
If no, then same goes for you, since you're new to Linux, so you'll have to learn when to use higher privileges (running things with sudo).
Second question. It's the difference between being a normal user or Administrator on Windows, so yes, as root you can do and run anything - you have the highest privileges (actually the same as the System user on Windows).