A long time ago when I (Scott) first started to take photos I was convinced it was the camera that could make me a better photographer. As my passion for photography grew, I immersed myself in countless photographic magazines and books. I studied the work from photographers like William Albert Allard, Mary Ellen Mark, and Gordon Parks, looking for information for insight into what camera gear they had used. My mistake was in believing that I have to use the same equipment at what they had if I wanted to take great photos. But even with all that great gear I found myself always coming back to the same camera and lens setup (An old Minolta SLR with a handful of lenses).... Why? Because it was a set up the worked for me. Many years latter Chase Jarvis coined the phrase "the best camera is the one you have with you". So basically it isn't that important what camera you use, what is important is how you use it. That said lenses do play a very important role but that is a topic for another day.
If you are using a modern camera, when you look through the view finder and press the shutter release button half way down your camera will make an exposure reading (as well as start to focus). In short it will look how much light is there available, this is called the EV (exposure value) and will be the basis for your photo depending on how you set your camera. The camera I use the most has 3 basic metering modes, naturally not every one uses the same Cameras as what I have so please refer to your camera hand book.
Your camera is only measuring the light that it can see, and may be fooled into measuring incorrectly in a variety of different lighting situations. The EV that the camera measures is also only a guideline based on the decisions made my an engineer in a lab somewhere, and my not be the correct value for the image you want to make.
The 3 aspects (settings) of your exposure are all in your control and changing any one will not only have effect the final image but also one or both of the other two settings e.g. I like to shoot with my cameras in A mode. I set the f/ (aperture) to the d.o.f that I want and I chose the ISO to match the lighting conditions (dependant on approximately what speed I want to shoot at) . If I adjust my f/ and leave the ISO where it was my camera will automatically compensate by adjusting the shutter speed. Remember you are taking the photo not the camera.
There are two great tools built into your camera to help you check and adjust your EV, the Histogram and the EV compensation both of which if used correctly will save you a lot of time latter when you are correcting/editing your images. Using the histogram can be overwhelming at first but is something that every photographer should understand to get the best results from their camera (and we will cover it at a later date)