The admin site is accessible through your private area. Once logged into the private area use the Admin Site link at the top of the page.
This is the key part of the site that you will use all the time to update your content. From here you can add, delete and modify the text that is shown to visitors on your site.
Note
The modules that you have installed affects what is visible here. More modules means there are more things you can enter. Removing a module does not remove the existing data from the database, it just makes it inaccessible in the admin site and via the web site.