Appearance options and plugins

I’m new to wordpress and trying to work on a website that belongs to a friend. My problem is that I want to add a plugin (I want to add icons that link to social media sites like Yelp), and all of the how-to’s I’ve read begin with clicking on parts of the administrative site that I can’t even find. One of the sites said to start by clicking on the “Appearance” tab, but it seems like it’s supposed to be located under the “Comments” tab on the sidebar. There is no “Appearance” button on the sidebar or anywhere else. All of the “Screen options” are already checked. What am I missing?