There is NEVER a stupid question! If you don't ask, how will you ever learn the answer?
The mode you choose is usually determined by what you are shooting. Every setting (aperture, shutter, ISO) has advantages and disadvantages...in photography, it's always give and take. Sometimes it's easiest to explain by example so here goes...
I shoot mostly landscapes with a K10D. I usually don't care about capturing fast action. I'm concerned mainly with getting as much detail as possible, large depth of field (as much of the picture in focus as possible), and precise exposure control to capture as much color as I can.
With this said, I almost always shoot in manual exposure mode (that "M" on your dial). I then do the following:
1)
Select the ISO speed that will give me the best quality. This is almost exlusively ISO 100 for me. In laymen's terms, the larger the ISO number, the faster the film and you can catch more action. But as you get larger on the ISO, you also get grainier pictures. Very low ISO will get sharp pictures, but you will need a very slow shutter speed which means any movement from your subject will blur.
2)
Select an aperture setting that gives me the most depth of field but doesn't cause diffraction (pixellizing). The larger the f/stop
number, the greater the depth of field, but also the better chance you will reduce image quality with pixelizing (depends on the lens where this will start to be noticeable). The smaller the f/stop
number, the less the depth of field. I usually pick something around f/16 or so.
3)
Select the shutter speed that will give me the exposure I need (meter). After I have selected ISO speed and aperture, it's simply a matter of looking in the viewfinder and rotating the shutter speed dial until the meter centers. When that happens, theoretically I have metered for proper exposure. Of course, I may end up over or under exposing a little to get the exact picture I want. Proper exposure by meter alone does not account for the human factor...that is, personal taste, artistic intent, etc.
Hope that helps! The best thing you can do is practice and experiment. Flyer is right in recommending that you use manual mode as much as you can to learn the techniques. Happy shooting!