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I am trying to understand something that is really confusing me.
If I want to make a captured image brighter I can do this in many ways. I can
Which of these options is the right one? How do I decide which of these things to do in which circumstances?
Each option has an effect on your image (and you can do a little of each as well), which you choose is a matter of choice.
Longer exposures, will create blur in moving objects (or in some cases make them disappear), this can be pleasing as in water around stones or a bit of motion blur with a baseball bat, or distracting (someone's face being blurred is rarely desirable).
Wider apertures decrease depth of field, great with some portraits, and other times a problem, e.g. some landscape work/macro.
Increase the ISO, depending on your camera how far you can go here will vary, but at some point all current cameras will become an ugly mess of noisy with low dynamic range if you go too far.
Flash, on axis (popup) you will get harsh shadows and a very flat look, as you learn to bounce and use multiple flashes you can shape your light -- controlling light give you tremendous power as a photographer. Of course it requires equipment and is not alway possible (high school coaches seem to freak out if I show up with a trunk of lighting gear in their gym).
Reflectors, hot lights, LED lights, moving your subject closer to lighting sources...
Which you pick, depends on what you are trying to create.
There isn't a right or wrong answer. It depends on what you need to accomplish, though certain options are less favorable. Raising the ISO for example, is generally an option of last resort. Raising the ISO means raising the noise floor which means that the quality of the image will suffer, but if you can't otherwise alter the lighting (such as using a flash or studio lights) then it may be the only option left to you if you need a particular shutter speed (due to motion) and aperture (due to depth of field).
In general, throwing more light at it is usually the prefered situation if throwing more light at it is possible and won't interfere with the shot you are trying to get. This gives the most flexibility in controlling your other factors without tradeoff, but it also won't work for many shots where you want to capture the natural lighting or where the scene is too big to practically light.
As far as changing the shutter and aperture, the aperture has the least flexibility in general since it impacts your depth of field. If you want a particular depth of field, that tells you what your aperture must be. You can cheat it a little by adjusting camera position and focal length, but that also impacts perspective. Shutter speed is the prefered option in cases where throwing more light at it isn't an option, but has a critical drawback that any motion will cause blur in the image if it is too fast in relation to the shutter speed. There are also some issues with noise accumulation in really long (10 to 30 second+) exposures on digital cameras.
If the shot can be done without motion, then shutter can be freely adjusted with minimal trade offs, but if there is motion in the shot, then shutter must be set fast enough to avoid blurring. When motion forces shutter speed and controlled lighting isn't an option, if depth of field isn't much of a concern, then aperture might be able to help some, but then as a last recourse, sensitivity (ISO) can be increased when all other options are exhausted.
