How can I make an image as my website background?

I am trying to make my own website, so I want a cool background. I found a good looking image on the internet and wanted to make that my sites background, but all the tutorials on internet didn’t help. Does anyone know the code to make an image your background?
I use notepad btw.

try looking at the background property on the documentation
or background-image property

Do you mean that I could have made a mistake at these properties? Because I think that my url for that image is wrong, but I don’t know how to fix that.

You asked how, you put an image as background using the css background-image rule

As you have not shown your code I can’t do any considerationg on that