CSS files use the file extension .css and you may use a notepad to create/edit this file.
Try typing this on a notepad:
body { background-color: red; }
..and save it as mystyle.css
Then on your html file, use the code below to reference/link to your css file:
<head>
<link rel="stylesheet" type="text/css" href="mystyle.css" />
</head>
Of course, both of your html and css must be in the same folder.
Hope this helps. :)