Welcome to SitePoint Michael.
Am not sure I know what you mean by "input the CSS", are you using a CMS or did you hand code yourself? If you used a CMS, the way you update, if possible, the CSS is through the control panel ( how you access this varies by CMS).
In general, you will what to find the style sheet CSS file and add your code to it. As I assume your site already has some CSS beign applied to it; it is likely a CSS is already linked. Find this file and add your code to it there ( dont forget to save)
Best way to link ( or find where this CSS file resides is to look within the HEAD tag of your page, it will look something like this
<link rel="stylesheet" type="text/css" href="[B]your/directory/styles.css[/B]" />
The text I HAVE BOLDED above is the path your CSS file.
hope that helps