Very basic child theme question

Ok, so kid gloves here people. I’m very, very new to wordpress…

Here’s what I have on my child theme:

-a blank page
and these empty files:

404 Template
(404.php)
Theme Functions
(functions.php)
Styles
Stylesheet
(style.css)
RTL Stylesheet
(rtl.css)

Here’s my question:
If I want to edit CSS, do I just open up the style.css and start entering CSS? Do I want to copy and paste from my parent theme before doing it, so I have it as a reference? It seems like people in videos/demos are always writing directly below the style that they’re overwriting, no?

The last thing I want to do is screw things up in an effort to improve things, you know? I’d rather leave styles as is than mess em up.