Before we can learn how to get out of the dreaded program mode and into the real meat and potatoes of the photography industry we first need to take a moment to learn (or refresh your memory) about how exactly a camera takes a picture.
The idea behind the film camera and the digital camera are very simple. Allow a certain amount of light to hit a certain sensitivity of media for a certain amount of time. In film, this was achieved by allowing light to strike tiny silver halide molecules that were suspended in an acetate backer. The silver halide would change physically when struck by light. The more light there was, the stronger the change was.
A long time ago when there was no color film, there was only one layer of these crystals which meant they reacted to the entire spectrum of light and color. As such, you could only have a one color image, white and shades of non-white. (Black and White). Soon after, it was discovered how to make film with layers of crystals that would only react to a specific color spectrum. And with this, color film was born.
With digital cameras the theory is the same, you have just replaced the film with what is (for now) a 3 layered sensor. Each layer detects it own color. And when you're done, you'll have an R (red) G (green) B (blue) image.
Now, how does all this relate to your Rebel XT? In order to begin to take photos correctly you have to consider the triangle of exposure:
1. ISO (or the sensitivity of the film/camera sensor)
2. The amount of light you want to let into the camera (the aperture)
3. And how long you want to let that light hit the sensor (the shutter speed)
Each one of these variables also has other uses, which we will explore in depth later. But, the main idea is once you know how to control these things, you can use them to do your creative bidding.