If you are editing an Offices and Services website, please see the documentation about adding content to groups.
When you are logged in, the Content interface is where you can create and manage content. To access the interface, click Manage and then Content in the admin bar at the top of your screen.
Click the blue +Add Content button. The next screen will present you will a list of available content types.
Select the content type that you want to work with and this will take you to the main editing page for this content type.
Please see our documentation on specific content types for more information about what options are available to editors in each.