Choose the element you wish to add content into by left clicking it. You will notice it gets highlighted with a red border. Also the controls change to match the options you have to work with for the element:
Once you left click into your element of choice, your tool bar changes. The new options are the things you can do when you are inside that element. Let's go ahead and click the "+" circled in red:
This opens up a new window of items that could be added into your element:
For instructions on how to work with each of these, please refer to the section on Elements and Widgets in the Pages and Content Basics article.