How to center - CSS/HTML

I have a question. How can i place everything in the middle? like this: http://moon-app.com/ You can see, everything is in the middle of the page. How can i do this.

And another question: there is default a border around what you make: https://gyazo.com/759e5072a1fb857ae998dae608bf9624

Does anyone have a script to delete this? Or how can i do this?

Thanks for al the help!