This wil undoubtly be the most important decision you make.  Choosing your theme determine how your WordPress site will look and how it will function.  It is important to take your time and evaluate what you like.  So go to https://<InsertYourNameSpaceHere>.wordpress.com/wp-admin/themes.php

You have many choices to choose from, and remember it's easy to switch themes later if you decide you do not like the one you pick out, just return to this page and choose another theme.  Clicking on a theme will give you details and tell you more about a theme. Here are the details for the Coraline Theme.  For our example we will select this theme for demonstrations. We will click "Activate" at the bottom.  Some themes will give you additional information when you activate them: 

Click on "Customize" to customize the theme for you WordPress Blog/Website.  Every theme will be different in what options you have to customize for your website.  In this case we are given the following: 

Along the right side, I have choices on Color, Header, Fong, Widgets, and Site Titles.  You should review each of these and read the information provided by the theme on what it will do.  You can see "Preview" in the left side as you make changes.  In addtion you can select PC, Tablet, or Phone views from the bottom center three buttons.  This will help you get perspective how it will appear on various devices.

We will cover Widgets in our next presentation.