We've seen a few great online tools for learning how to use the manual settings on a camera before, but Photography Mapped is a new web tool that's worth playing around if you're new to manual photography.
With the web app, you can adjust the lighting environment of a scene with a little helicopter, then from there, change your aperture, shutter and ISO speed. You'll see what the depth of field will look like as well as any motion blur and noise below each setting. When your experiment is all set up, click the "Take Photo" button and you'll get an estimation of what the shot would look like as well as some notes on why it looks the way it does. If interactive isn't really your thing, there's an infographic version as well, and a pretty cool looking print that's available for purchase too.