A high-key photo is basically white on white. This style of photography conveys a feeling of lightness and clarity. Typically what happens is the camera wants to make the white of the photo - without dark areas to 'balance' the exposure - too dark. So your white's become gray. You may have seen this particularly when shooting a snowy field on a gray day. However, exposing for high key is fairly easy.
With an in camera meter, you can measure the light hitting a white area, and open up two stops - such as changing the aperture from ƒ11 to ƒ5.6.
Exercise: Put a large piece of white paper or white fabric on a table beside a large north facing window, place a white object such as a Styrofoam cup or egg in the middle.
Take a picture with the exposure that the in-camera meter suggests. You may have to use a tripod to keep the camera still if the shutter speed is too slow.
Next, meter off a white area and open up 1 and 1/3 or 2 stops - as described at the top of the this page. Compare the images.
This can work well for other scenarios where exposure is a tricky thing - like weddings where the bride is wearing white and the groom is in black. Quite often, the dress winds up blown out with no detail in the dress. By metering off the dress and opening up two stops you can prevent this from happening.
You can also use a hand held meter to measure the light hitting the subject, which will be more accurate than the in-camera meter.
Note: Be aware that some digital cameras have exposure compensation built in to prevent overexposure. If exposure is too bright the highlights could be "blown out" and detail lost int the brightest parts of the image. By artificially "darkening" the image, the camera makers try to make sure the exposures aren't too bright. This doesn't affect all cameras but it does seem to be the case for some. That means that the exposure needed in lessons 2, 3 and 4 may be slightly higher than suggested in the lessons. You might use the "expose to the right" method.