To connect your HTML file to CSS, include the following code in your HTML's <head> section. Remember, the <head> section contains the attributes and general settings of the HTML page.
<head>
<link rel="stylesheet" href="nameOfYourCssFile.css">
</head>
In the example above, the html and css file live in the same folder, so I don't have to specify the CSS's files location, for example: /css/styles.css.