hi ceddy, these are the fundamentals of photography. you might find Michael's articles on the main page helpful. Or any photography book might serve you well. however, i'll try to list down the points for you for a quick start. i still remember how i started to learn all these stuff, pretty confusing at first too.
let's leave aside the aesthetics of photography (that i'm in no position to offer any help) and only talk about tech part.
a photo is usually properly exposed (unless you want something special).
exposure = amount of light x ISO............................................................... (1)
amount of light = aperture x shutter speed .............................................. (2)
therefore:
exposure = aperture x shutter speed x ISO.............................................(3)
i assume u know what ISO, aperture and shutter speed are.
the mordern cameras know (though sometimes can be fooled) what the proper exposure should be.
Av (aperture priority) mode: you decide what aperture you want to use, camera caculates the shutter speed for you.
Tv (shutter speed priority) mode: you decide what shutter speed you want to use, camera caculates the aperture for you.
M (manual) mode: you decide both aperture and shutter speed.
P (program) mode: camera decides both aperture and shutter speed.
ISO is always set by the photographer (as far as i know, there's no ISO priority yet, a nice feature though)
so now look at equation (3), on the left, camera knows the exposure *, on the right, ISO is always decided by you, there are two left: aperture and shutter speed. if you wanna control aperture, go Av mode**; control shutter speed, go Tv mode***. control both, go M mode****; control none, go P mode*****; it's that easy.
* (i). what camera measures is light, exposure is caculated from amount of light and ISO. you can measure light with a seperate meter too to be more accurate. (ii) for canon cameras, there are three build-in light metering modes. choose the one that suits your need. (iii) exposure is under your control too. to over or under expose is do-able under Av, Tv & M mode.
** aperture decides the Depth of Field of your picture. when you shoot landscape, you may want everything sharp from foreground (a flower) to background (mountain), it's a deep DoF, small aperture is needed; when you shoot your girlfriend, you may want to blur the background and let her face pop-up, it's a shallow DoF, large aperture is needed. In cases like these, you need the control over aperture and you use Av mode.
*** shutter speed decides the length of time your shot is taken within. when you shoot moving objects like birds, or in sports. you need to "freeze" the motion to be sharp; when you shoot night scenes or night sky, you might need a long exposure for seconds even minutes to collect enough light. In cases like these, you need the control over shutter speed and you go Tv mode,.
**** in certain cases, you want full control of everything. go M mode.
***** in P mode, camera decides everything for you. you camera now works like a point-and-shoot. camera chooses aperture and shutter speed according a pre-set program. it works decently if not under extreme light conditions.