Hey there Ninja,
lets give you a quick crash course.
The main three things that your going to be working on are: shutter speed, aperture (which is the f number) and iso.
I'll just give you a quick run down of what each does, and when to use it.
-Shutter Speed- As you already stated this controls how long the shutter is open for. You camera probably ranges from a 30 second to 1/4000th of a second exposer. You would use a shutter speed of 1/500th and above for action photography. While on the other hand, you'd use a 15 or 30 second exposer for night photography (just one example).
-Aperture- Mainly this controls the DOF (depth of feild).
DOF controls how much of the shot is or isn't in focus. Using a higher F# such as 18 (smaller opening), will result in the majority of the photo being in focus, however less light is being let into the camera, so you have to compensate for that with either the shutter speed or iso. If you only want the subject to be in focus then your going to be using a lower F# 2.8 or so(larger opening). Your most likely not going to have a F# of 2.8 or less until you start spending some serious money on glass. The reason lower F#'s are wanted (and expensive) is because it allows you to shoot in lower light conditions, or allows you to gain a faster shutter speed when needed. However it's not always beneficial to shoot wide open. The lenses sweet spot is probably around F# 8.
-ISO- If you were familiar with film, you would then refer to this as ASA or the film speed. I tell people to think of it as "Fake Light", since that is effectively what it is. Since your using a Rebel XT (if I'm not mistaken) then I would recommend staying with in the range or 100-800. If you try to boost it higher then that, you'll most likely start to get very noisy shots. Granted some times it's needed. If your shotting outdoors in good light then keep your ISO as low as you can get away with, but if your shooting in less then adequate light or need the extra light for a faster shutter speed then your going to need to bump the ISO. However, resort to ISO above 800 for a last effort. First make sure your using your lowest F# and that the shutter speed isn't way above what it needs to be. Also, if you have a good flash unit, do let it go to waste.
Hope this helped.
If you need anymore help just ask.
Bob
Heres a quick example for a longer shutter:
(Photobucket killed the quality so don't mind that)
This shot was taken using:
Shutter Speed- 30 seconds
Aperture (F#)- 9.5
ISO- 200
I wanted to keep the noise down so I used a iso setting of 200, so in order to properly expose the shot I then needed a long SS, in this case 30 seconds. Now I could have lowered the F# to say 3.5, however the sweet spot for my lens is around 9.5, so I decided to keep it there.
I hope that settled some confusion.