Simply go to Content>Article Manager and click on the 'new' button at the top right:

Make sure to catgegorise your new article by choosing something in the Section and Category fields. Once you've finished, just click 'Save'. Please note that creating a new page will not necessarily make it appear in your website. You might have to create a new menu item to link to it. Alternatively you can set it to appear on the front page. See the image above in the 5th column? The first item in the list is enabled on the Front Page (see the green tick). If you want the other items to appear on the front page, click the red circle with the cross and this will change it to a tick ie it will publish it.