Using the admin menu, select Content>Add Content> Page (it might say Basic Page).
You will now see a page where you can enter all of the information for your new page. Fill in the Title and Body Content.
By default, your URL will automatically be created based off the title of your page. If you would like this page to have a specific URL, read the instructions for creating a URL/alias.
If you would like this page to appear in the menu, read the instructions for adding a page to the menu or read the instructions for adding a page to the sub-menu.
Click Publish.