How do you create the style.css file for our own projects?

You need to create a new folder in the /themes/ directory, call it what you like but something logical. You then need to create a new file name style.css inside of that folder (or create it locally and upload to the folder) - the start of the style.css file needs some information so it can be properly declared a child theme - the code you need for this is below - edit it accordingly.

 Theme Name: The name of your theme
 Description:  Description to be displayed in the WP back-end
 Author: Your Name
 Template: twentyfourteen
 Version: 1.0.0
*/

@import url("../twentyfourteen/style.css");```

Ok, thanks. So the most important parts are the Template and version # of template and the import part right?

So only if you are making changes to the css do you need to create a child theme right?

You always need the parts included above - you can create the child theme without these, but it isn't advised (actually you don't really need the version number, but i always use it). You MUST include the @import otherwise the basic styling from the parent theme won't be used in your child theme.

You create a child theme for making edits to a theme, yes - this way when Wordpress is updated your CSS changes are in tact, since they're located in a child theme folder (i.e. not a core part of Wordpress, for which files are overwritten when Wordpress is updated).