Av -You change the aperture but the camera controls the Shutter speed (you can still focus manually).
Tv - You change the shutter speed, but the camera controls the aperture (you can still focus manually).
P - The camera controls both the aperture and the the shutter speed, but you control everything else
C- Custom. You can pre-set the settings and save them.
M - Manual. You can manually adjust all settings on the camera.
Here's a brief tutorial on some of the settings:
To best understand the settings for any particular shot, you need to take into consideration all the settings of your camera. Each setting has it's own effect, and adjusting these will give you the desired type of picture you want. Here's a brief list of some of the settings....
Aperture- This adjusts how much light goes into the actual sensor. The more light, the more information it has to process. However, too much light in a bright setting will wash out the image.
ISO – This setting tells you how sensative the sensor should be. The higher the ISO, the more sensative it is. Too little ISO in a low light setting means a dark image. However, the higher the setting, the more noise the picture gets.
Shutter Speed. - Although most digital cameras don't have a physical “shutter” that opens for the picture, they do have a “digital” shutter which determins how long the sensor is actually picking up information. A fast shutter means it has little time to work with, and in low light situations can be bad. However, too high of a setting will lead to anything moving being blurry. If you're holding the camera with a high shutter speed, EVERYTHING is moving according to the cameras lens. If you were taking action shots, a high shutter speed would give clearer shots. However, at night, it wouldn't be able to process much information and would lead to an under developed shot.
In essense, there is no one setting for your camera. The settings will change from place to place, and sometimes shot to shot. In high light settings, you might try a lower ISO and faster shutter speed, then adjust the aperture to get the desired brightness. At night time, you can TRY a higher ISO (as low as possible), and raise the aperture and shutter speed as needed. If you're taking a picture of a moving object, you would want to keep the shutter speed low to avoid a blurring effect, but the overall quality will diminish. If you're taking a picture of an inanimate object, you might try raising the shutter speeds, lower the ISO and adjust the aperture to the correct settings.
As a final note, it just takes PRACTICE. With enough practice, you'll begin to have an idea of what settings you need to change before you turn your camera on and everything will flow. With enough patience and batteries, you'll end up with photographs you'll truly enjoy.
I hope this helps.