You must use the editor which is available under the Appearance menu on the left side of the dashboard. When you click on the editor, you will be taken to the Stylesheet. From there you can add your CSS that you desire. If you plan on editing HTML and PHP, you must click on the files towards the right after you have clicked the editor link. Look for the header.php, footer.php, and so on. Those are the files that are loaded whenever a new page is requested by a user. In addition to the header and footer files, a good theme will have different template files. For example, it might have a homepage template. If you want to change the look of your homepage, you must go to the homepage.php file and edit it to how you would like. You still access that file the same way as you get to the stylesheet, through the editor menu. Look at the page template that you're using and make sure that you're editing template or it can be disastrous.