Once you have created the website and set your users, you are ready to start updating the site. Simply click on the Login button at the bottom the site details dropdown to get started. This will log you into the root of your new website. From there, you can:
Access to these functions are through Admin and Edit links generally at the bottom right of each page in the website (but you can put them where you want, or make them look however you'd like)
See help.editmy.website for help on the updating of your sites.