To create a new Page, use the administration menu at the top to go to Content -> Add content -> Page.
This will take you to a blank Page Editing screen, where you can put in your Title and Body. You can also add content to other fields, detailed elsewhere in this manual.
When creating a new page, it's important to remember to add your page to your site's Menu. Before saving, click on the Menu settings tab on the left, check the box that says Provide a menu link, and use the drop-down to select where in the menu you want your Page to live.