ISO is more of a “film speed” thing. Higher the ISO the more noise that will be in the photo, but the more light it will let in. So if you have a high ISO you can shoot in lower light more easily. A lower ISO will give you a more saturated shot but require a longer shutter speed to compensate.
Shutter speed is how long the camera shutter stays open allowing light to hit the sensor. This is all measured in fractions of a second; 1/60th is about the average slowest shutter speed you can take handheld without have a blurry subject. Some can do a slower shutter speed fine, others cant.
Aperture is how much light the lens allows to enter the camera through the lens. It works like the pupil in your eye. A smaller aperture number allows more light and a larger number allows less light. A smaller aperture number will give you more background blur while a larger aperture number (such as f/22) will have everything in the photo in focus and very little blurred.
All three effect each other. If you have a large aperture, like f/1.2 its going to allow more light to enter the camera, so this will have to be compensated with a lower ISO and faster shutter speed. If you want a longer exposure time, a lower ISO, and larger aperture will be needed.
When you look through your viewfinder you will see:
The scale on the bottom that is in the middle that looks like: -2…-1…0…+1…+2, is your light meter. When you have the camera on full manual mode, this is how you adjust your exposure/shutter speed. A setting of “0” or right in the middle on the big divit mark is a proper exposure based on light and surroundings. It will change based upon where you aim it. A negative side of the scale is underexposed (darker image) and the positive side will be overexposed (lighter image).
The way I learned is just putting the camera on full manual mode and shoot. It takes practice and patience, you wont learn it overnight.
Here is a little more info that can help you too: http://forums.mycanikon.com/showthread.php?t=1939
http://forums.mycanikon.com/showthread.php?t=4284