ISO changes the sensitivity of the film (censor) to light. The higher the ISO, the less light you need to make a picture. Lower ISOs are better. They create less noise and pixelation in your photos allowing you to make sharper photos. With film, ISO is standard. with digital it isn't. Nikons ISO 100 is the same as cannons ISO 25. You just need to do test shots with your camera. The darker the subject the better you will see the amount of noise being created by the higher ISO.

Also try to watch out for super long exposures. CCDs cool poorly and can heat up unevenly. This will cause a phenomena known as blooming where your hilights turn pure white (255) and then the heat from that causes the surrounding pixels to also turn white.

With aperture, the larger the number of your aperture the smaller the physical hole is, and the less light gets into your camera. so F22 will have almost everything you see in focus, but you need a lot of light to get the right exposure. But F5.6 needs much less light to make a proper exposure, but much less of your image is in focus.

White balance tells your camera what white is.

Ok, now with your exposure. When you look through the lens and it tells you that you have a proper exposure, what it is actually telling you is that whatever you are looking for is being properly exposed for 18% grey. If you are metering white, it will expose it for 18% grey. What you need to do is change your exposure to make your whites white. Either in the camera, or in a program like gimp or photoshop.

Fun tidbit- the reasons cameras expose for 18% grey is because that is the proper exposure for Caucasian (white) human skin in film cameras. Digital cameras just took the same idea and ran with it instead of doing something new.