I don't know where to start. Oh yah I know nothing about your camera model,
I'm a Canon guy.
That said:
AP is Aperture Priority, it means that the aperture stays where you set it and the camera adjusts the shutter speed to correct the exposure. The opposite is SP-shutter priority where the shutter speed is set by you and the camera adjusts the aperture for the corect exposure. For us AP is the way to go because it gives you control over the depth of field. That is assuming you use a tripod and you should be using a tripod because some of the shuttter speeds can get longer than is acceptable for a hand held shot. F8 is an f stop number. The larger the number the smaller the aperture, the less light that is let into the camera and the greater the depth of field. All you really need to keep in mind AT ALL TIMES is that there are only four perameters that control the exposure of a photograph. 1) is avalable light, 2) is film speed, 3) is shutter speed, 4) is aperture. Light, shutter, and aperture control the amount of light that hits the film and makes a photograph. Film speed determines the amount of light NEEDED to make a photograph.
Another noteworthy point, underexposed is too little light and the picture is too dark. Too much light is overexposed and the picture is light and washed out. This could go on for hours but I will stop here for now. Generally speaking alittle underexposed is good and overexposed is just bad.