16×9 is the aspect ratio of widescreen tv’s, hd video, and increasingly the aspect ratio of online video. Importantly, this ratio can exist is an whole variety of resolutions. In other words, 1920×1080, 1280×720, and 640×360 are all the same 16×9 aspect ratio. For your first project you will be rendering a 640×360 video, a size that is both easily posted online and easy to render.
And to help you with your projects I have uploaded a .blend file that has the final output settings for the project. You can start your project with this file or copy over the settings to your .blend file. Again the final output is to be a 640×360 .flv video with mp3 audio. The example .blend is 1500 frames or one minute in length.
Also here is a blank 1280×720 px image that you can use as a starting point in photoshop.