In order to use your account to full capacity, you will need to configure your site settings. Once you have your domain, go to Settings > Your Site Settings > General Settings and enter your domain URL under Site URL. Click on Generate DNS Entries to see exactly what DNS entries you will need to configure.
Once you have configured your DNS, click on Validate to verify that everything is set up correctly. Remember to click Save at the bottom of the page to keep your changes.
You can now move forward to configuring your DNS settings to ensure deliverability for your clients.
Tip: Internet Explorer has issues with domains containing underscores. For your own safety, do not use a domain that contains an underscore.