ok so I understand the different setting and how they work (SS, Aperture and ISO) , but im not really sure which ones are the appropriate settings to adjust when shooting. Is there a certain guideline or rule of thumb to follow?
What you should eventually find is that every situation, every scene and every idea you have is unique in some way. After that realization sets in, two things will happen if you are typical of student photographers.
First, you will somewhat develop your own methods of working with your camera and with your subject.
Two, you will see each leg of the exposure triangle as contributing to holding up the other two.
If you think back to your school years and how you were taught to work with triangles, you'll easily see changing one value affects the other two. This is how the exposure triangle operates as you adjust your camera to accommodate the vision you have created for "this" scene.
Experience will teach you which value of the triangle is the most relevant to your perceived photo. Of course, you must develop the "vision" of a photographer before you can understand the value of each exposure value in turning out that vision.
In other words, what are you shooting? How do you see the scene playing out, whether it's a scene that is rapidly changing or a scene that is essentially static?
What are the lighting conditions? Light is your most significant material. Your camera provides the tools to work with and to sculpt the light.
How will you work with the existing light? Should you add supplemental light? Should you change your location or the location of your subject?
Eventually, you will have these (and other) questions in your head as you approach a potential shot. The answers you produce will largely inform how you will "work" with your camera.
Low light suggests "these" answers. Bright light says you will most often use another technique.
Large depth of field suggests "this" answer.
Capturing or stopping movement indicates another answer. Suggesting motion requires yet another technique.
All of this is what you are (eventually) calculating in your mind as your vision of the photo develops from the scene in front of you.
As I mentioned in another thread recently, I was reading an article by one photographer who suggests ISO is the value you set first and adjust last. In many cases, this makes sense.
You should be very aware of the prevailing light and the conditions it presents to you as a photographer. General lighting conditions will quite often make ISO values the most important and easily the most accessible value at your disposal.
Overall, ISO is in many ways the most benign of the three exposure values. Modern DSLR's are capable of high quality at much higher ISO values than would have been possible even five years ago. The digital noise we associate with higher ISO values is relative to print size. If you are printing at, say, 5 x 7, noise is less an issue than if you are printing at 20 x 24. There will be little noticeable effect of raising ISO to a reasonable value in most student situations. And not raising ISO will severely limit your use of the other two exposure values.
IMO a student photographer can do little wrong by setting your DSLR to "auto ISO" and then limiting the maximum value to aprox. 1200-1600. This will give you the greatest latitude when working with your other values. As you become more experience with your camera and its capabilities in various setting, you can adjust that maximum value up or down.
Of course, always pay attention to what you camera is doing and make shot by shot adjustments as you feel appropriate.
Quite often, your use of either aperture or shutter speed will depend first upon your selection of how to meter the light within the scene. Now you are beginning to refine your vision of how the final product will look based on knowledge and experience.
Of course, the very simple rules of closing down the aperture to affect depth of field hold. As does the concept of opening the aperture to allow in more light.
Fast shutter speeds are usable in brightly lit situation whether that light comes from existing light or from supplemental light sources. Faster shutter speeds will freeze motion and avoid camera shake blurriness in your shot.
Slower shutter speeds will allow the available light to enter the lens for longer periods of time. They can be used to imply motion but they are also somewhat less usable to the student unless they have developed good camera techniques or they mount their camera on a tripod or similarly stable mount.
As you further develop your skills, you'll want a few accessories for your kit which will make these various adjustments more flexible in their application. They will provide more options for you to consider and each option may change exactly how you work for any one scene.
Flash units, tripods and filters all fit into this category. Each changes how you think as a photographer. How you think affects how you work.
One technique which you may find useful is to sit down for an evening or two with your camera and the owner's manual. Study metering modes. Take multiple test shots using various legs of the triangle with various metering modes.
Understand focus points and how to change them. Notice the changes each shot makes to the idea you have of how to photograph a common object.
Repeat the exercise in bright sunlight and then in the fading light of sunset.
(Notice too that this same time of day will change significantly as the months and seasons pass. As a photographer you need to become sensitive to these changes and how they will affect your photography.)
Work in both aperture priority and shutter priority. With a DSLR all of these shots are free and you can save or delete any of them. With your owner's manual at hand (or a handy cheat sheet for your specific camera;
Canon EOS Rebel SL1/100D For Dummies Cheat Sheet - For Dummies, you can easily make the changes you feel are needed to create the image you have in your head.
Spend some time on line watching videos. Place your camera's make and model into a search engine along with "tips and tricks". Now you will have instructional videos and lessons based upon a more experienced photographer walking you through your specific camera and how to get the most from that gear.
Walk out to a point in your neighborhood where you can see a busy intersection. Take test photos using the various legs. Try to stop the motion of the cars as they pass through the intersection. Try to suggest the speed of the cars as they pass. If you don't yet know how to do that, refer first to your owner's manual and then to your videos.
Keep a photographer's journal of what you are going to shoot, your ideas for that specific shot and the prevailing lighting and weather conditions of the moment. You can use the numbering system of your camera to keep track of each shot as it relates to your journal. Many modern DSLR's allow a few seconds of video with sound to be recorded before you take a shot. You can use this to keep your journal notes.
Then you sit down with your camera and you look and you compare. The more experience you have, the more capable you will be at seeing the differences between all of your test shots. Relate these differences to the section of the owner's manual which explains how to set the camera for that effect.
In time, you will begin to associate your experience with past shots to how you are seeing a photo as you approach a scene.
There really is no substitute for experience in this hobby.
