The main window shows a draft preview which is useful for editing, but the quality is too low for production. That's why you need to render the scene when it is done. It is also useful to render the scene in draft quality to see if it looks exactly as you need, because the editing preview picture may differs from the rendered one.
To start the rendering click the "Render" icon in the toolbar:
You will see the settings window, where you can adjust some parameters. Then the rendering itself that may take a while, depending on the settings and the scene complexity. Finally, you'll see the rendering results and will be able to save and share the results.