Once again I am going to keep this as simple as I can so I can give you everything you need to know to shoot in manual mode without overloading you with every technical detail. Basically the aperture is a device in your camera that can be opened wider to let in more light or closed tighter to let in less light. Here's the thing that is a little confusing about aperture: the lower the number, the larger your aperture, which seems counter intuitive, but once you accept that fact it's not so bad. So an aperture setting of f1.4 means your aperture is open very wide (that's almost as wide as you can ever go) where as an aperture setting of f16 means your aperture is closed down pretty narrow. You may have noticed there's always an f in front of the aperture setting.
Keep in mind there are tons of choices between the two on my chart above. Here are some rules of thumb I use:
If I'm shooting one subject (or two subjects on the same plane) I often shoot wide open around f1.4. (Exceptions to that coming later in this post).
If I'm doing a family or group where subjects are going to be in two rows (on different planes) I usually shoot between f4 and f8 and I always focus on the front subject.
If I am shooting a large group with three rows or more I'll shoot at at least f12 or f13. It's always a good idea to take a picture and then zoom in and look at everyone's faces at 100% to make sure they are all in focus. If not, increase your aperture setting.
This photo is a good demonstration for keeping your subjects on the same plane. I was shooting wide open here at f1.4. My boys were actually sitting right next to each other so they were on the same plane when I was standing directly in front of them, but then two things happened.
First I moved to the side and you notice my oldest son became much closer to the camera than my younger. He also leaned forward at the moment which made it even worse. You can notice I was focused on my youngest son and that worked fine until I moved, putting them on different planes and also my son lunged forward, putting himself even closer to the camera.
Now you could close down your aperture a little more to compensate for this, but in this case, I was shooting in low light (a hotel room) so I needed the extra light of the wide open aperture. In that case I just needed to stay directly in front of them to keep them on the same plane and thus in focus.
How do you choose an aperture setting?
Your first question should be, am I shooting in low light? If the answer is yes, then some of your creative freedom is going to be taken from you as you may be forced to open your aperture up to get enough light, as I did with the photo above. When I'm shooting in low light I usually open my aperture all the way up.
If you have enough light (you are outside, right by a large window, have really really good lighting in your house, or have studio lighting) you have the freedom to choose your aperture based on what you would like your depth of field to be. (again, shallow depth of field that comes from wide open aperture = blurry background)
Wide open aperture of f1.4 = shallow depth of field/blurry background
Small aperture opening of f16 = deep depth of field/everything in the photo is in focus (the subject is still more in focus than the background but for the most part it looks in focus).
As I mentioned earlier I often like shooting with my aperture wide open. But there are exceptions to that and that's because there is one other thing besides aperture setting, that effects depth of field. Don't worry, it's nothing too complicated. Basically, the closer you are to your subject, the shallower your depth of field will be.
Both of these images were taken with an aperture setting of f1.4, I just moved farther away from the cake pops in the second photo. Notice how much more in focus the back cake pops are when I moved back a bit? In the first photo the depth of field is so shallow only one cake pop is in focus and the rest are pretty blurry.
The lens that came with your camera body probably only lets you open your aperture as wide as around 3.5. Lenses that open really wide (letting you get creamy blurred out background as well as shoot in low light situations) cost a pretty penny, but can be worth it if you're serious about wanting to be able to shoot indoors without using that dreaded flash.
One of my favorite lenses is a Nikon 85 mm f1.4 (the aperture opens all the way up to 1.4). Another, much less expensive, favorite is a Tamron 28-75mm f2.8 (the aperture opens up to 2.8). Note that if a lens gives a range for it's aperture (for example a 18-105 mm f3.5-5.6) that means as you zoom in your aperture will not be able to open as wide. It can open to 3.5 at 18mm, but only to 5.6 at 105mm.
You guessed it, go to youtube and search for "how to change the aperture setting on (insert your camera model here)" Watch the video and practice changing the aperture up down and all around. While you're at it, go back and refresh your memory on changing your ISO too.
Okay so you may have noticed that aperture is a bit more complicated than ISO. If you have any questions leave them here in the comments and I'll do a follow up Q&A post before I jump into shutter speed. If it's making sense to you then go celebrate because aperture is the most complicated of the three elements to grasp (in my opinion anyways).