Once you log into the CMS app you will see the Home Page of your website.
You will notice a desktop and a mobile icon right in middle of the page, by default it is on the desktop, to view the website in mobile view click on the mobile icon.
Apart from that you can choose the language you want to preview your site in from the language drop down.
To the left of the page you will have the Pages and Widgets. You are on the Pages by default, to work on the widgets click on it.