You can only create new content on pages where you have permissions to do so. If you have such permissions, you'll see a yellow toolbar at the top of the page with Contents and Settings tabs.
To create a new article on a page, click Add on the Contents tab. In the menu that appears, choose Article. This creates a new article which, much like a blog entry, will appear at the top of the that page.
You must enter a title in the Title field.