So now that we know more about aperture and shutter speed and what they can do for a photo - it's time to answer those ever popular questions: which one do I choose? How do I know which setting to choose first??! The thing to remember is your choice of settings will always depend on a couple of things: 1) the look you want for the photo 2) what type of lighting situation you're shooting in and 3) what you're shooting.
Now you should know that just because you want to use certain aperture or shutter speed does not mean you'll always be able to shoot with it (with natural light anyway.) If you're shooting in a very low light situation and want an aperture like f/11 with a shutter speed of 1/200 and you don't want to use flash... well more than likely it's not going to happen because that's a small hole and you'll have very little light. You'll get an underexposed image.
It's the same thing if you are outside shooting on a bright, very sunny day and you want to use a wide open aperture like f/2, but don't have a fast enough shutter to stop too much light from pouring in that big aperture. You'll get an overexposed image. In either of those situations you'll basically have to decide to compromise the setting you initially wanted to get a correct exposure. Or you can always wait until you have more or less light, add flash, etc.
That being said, people still wonder which settings to use in a certain situation. There's no magic setting I can tell you. I can say anytime you're in a low light situation you'll usually always shoot with the biggest aperture you have and a higher ISO (unless you add flash.) If you want to freeze motion you'll need to either use a fast shutter speed or you can use flash (flash freezes motion.) It will always depend on the situation, the lighting and what you want.
Here are some examples...
settings: f/4, 1/125, 100 iso
In this family portrait of my cousins, aperture was the most important choice. I wanted an aperture that would blur the background some but I needed the siblings in focus. I chose my aperture first, then adjusted my other settings accordingly. When shooting living things I also like to keep my shutter speed at least 1/100 or faster, so after choosing my aperture first I will bump up my ISO to get my shutter where I need it to be. In this case, a 1/125 shutter speed was sufficient as they were posed and being still.
settings: f/2.8, 1/200, 200 iso
In the photo above, the race has started! I knew I wanted to capture and freeze the motion of the riders so shutter speed was my obvious first choice.
(photo right) settings: f/2.5, 1/3200, 160 iso
Aperture was my first choice for these wildflower photos. For the photo on the left I wanted a smaller aperture to get the entire field in focus - while the photo on the right I chose a big wide open aperture to focus on just a couple of bluebonnets in a blur of surrounding color.
settings: f/10, 60 seconds, 100 iso, tripod
Anytime you want to try light painting (as in the image above) shutter speed is your first choice. Very slow shutter speeds allow enough time for you to run around and draw with light.
settings: f/22, 30 seconds, 250 iso, tripod
If you want to freeze or blur motion, shutter speed should be your first choice. A fast shutter speed is a must for catching fireworks. If you want the fireworks to leave trails of light then you'd want to use a slower shutter speed. In the photo above I used a 30 second exposure... the fireworks move really fast so we still get plenty of definition in them. Notice the trail of lights on the bottom right from a car driving by. It was moving slower and so we had this continuous trail of light. If you wanted even more definition in the fireworks (less blur) then you'd need to use a much much faster shutter speed.
setting: f/5.6, 1/200, 500 iso, bounce flash
Since I wanted both of the wedding rings in this shot to be in focus - aperture was the most important choice here. Anytime you shoot in a macro setting the shallow depth of field is intensified even more. This means a smaller aperture is necessary to get both rings in focus. While f/5.6 is not a super small aperture, it's still not a wide open aperture so there's more depth of field in this shot (with just enough blur.) Also since this image was shot inside at night, I had to use flash. The flash was bounced off the ceiling with an external flash.
settings: f/3.2, 1/250, 200 iso
Anytime you want to focus on your subject and make it stand out, while getting a nice amount of background / foreground blur - aperture will be your first choice.
settings: f/5, 1/250, 2500 iso
For this engagement photo at Baylor University, the shutter speed was important to get and freeze the motion. So I chose my shutter speed first then set my other settings accordingly.
settings: f/2.8, 1/100, 2500 iso
One of my favorite photos is this moment of Audree, before she walked down the aisle to get married. Because she was standing in a wedding chapel that had pictures of other brides behind her, I wanted to focus only on her, and I chose a big wide open aperture to get this look.
settings: f/2.8, 1/100, 3200 iso
Another photo of Audree with her dad during the father daughter dance. After taking several photos with flash, I wanted to capture the ambient light in the moment. So my priority was aperture first (wide open) and a very high ISO to help get both shots without flash. Notice the grain in the images- I was willing to use a high ISO for both of them to get the look I wanted.
settings: f/4, 1/160, 100 iso
When shooting Cari's bridals, she had mentioned a shot where the bride was spinning in her dress. To get this shot we were going to need a fast enough shutter speed - so the shutter speed was our first choice. Then we adjusted our other settings accordingly.
If you're new to shooting in manual - keep practicing! Next time you're out shooting be aware if you want to choose aperture or shutter speed first, depending on what you want. Aperture will always be my favorite choice but I do love shutter speed and all the fun stuff you can do with it.