Cascading Style Sheets are an sister language to HTML. Where HTML contains the content, CSS contains ideally all formatting that differs from the default given by the browsers. For instance, you can define in CSS how large and what font a first degree heading should be, and so on. This page uses CSS. Have a look on how the same page looks without CSS. Again there is a Web page allowing you to create your style sheet very easily. And here is a tutorial (from CPT106) on CSS.
is more than adding style sheets. There are good web pages giving advice: an old manuscript by Tim Berners-Lee, a very nice page giving also another introduction of HTML, A web style guide from Yale University, and the Killer web sites page.