Upcoming articles for your newsletter will automatically populate after you have added Sources. Once articles populate in Upcoming Articles, you have the ability to toggle an articles off that you would not want sent in your upcoming newsletter.
You can also manually add an article that you would like to appear in your newsletter for all contacts. Go to Content → Add article and input the url of the article that you would like included. Once added, the article will appear in your Upcoming Articles page and be included in each contacts newsletter.