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.
Use the text editor to add text, images and links to the body of your article. See Use the Editor section of the general Help FAQ for details on the editing functions available to you.
When you are done editing, press Save to finish.