Step 1: Your theme
Your theme is a default WordPress one by default. To change that, login to WordPress and click on the "Appearance" link on the left. This will show you all the available themes.
The most popular theme is Tweak Me v2. If you want to activate that, scroll down until you find it, then click the button to activate it. The settings for Tweak Me v2 are then located in Appearance > Theme Options.
Step 2: Set up a navigation menu
You'll want a navigation menu on your site, of course! To set one up, go to Appearance » Menus and create a new one. From there, you can add your pages to the menu and save it. Make sure to assign the menu to a menu area. In Tweak Me, this means checking the box to assign the menu to "The Main Menu".
Step 3: Plugins
Start picking and choosing plugins! To get you started, you'll want to set up your Comment Reply Notification settings. This plugin will automatically notify your commenters when you reply to them.. but you need to get it set up. Navigate to Settings » Comment Reply Notification. Switch the setting from "Disabled" to "Anyone replies" (recommended). Then save the settings. That's all you have to do here!
Next, go to Plugins » Akismet. The Akismet plugin will block all spam on your site. But before it will start working, you have to create and enter an API key. This is completely free! Click the button to "Create a new Akismet Key" and you will be taken to Akismet's website. You will have to sign in with a WordPress.com username and password. Note that this username and password is separate from your Book Host login. If you don't already have a WordPress.com username, you can create one for free on Akismet's website.
Then follow the instructions to get your API key. When prompted, select the "Personal Blog" option and you can take the price down to $0 (free).
When you have your key, return to your WordPress site and enter it on the Akismet settings page.
Now you can navigate to the main Plugins page and start activating the ones you want to use on your site.