The idea of aperture is a fun one and the priority beginners focus on most. The reason being is that aperture affects the depth of field. Otherwise known as how blurry the background is. But what is it also affecting that you can't see as well?
Aperture is a major part of the exposure triangle. Like I said in my post on the basics of photography it affects shutter speed and ISO. When you change your aperture it changes the exposure. SO, if you want a really blurry background and a shallow depth of field you will have to adjust ISO and shutter speed to expose properly.
The way it works is inverse to how you describe it. Aperture can be kind of confusing at first. The reason being is the higher numbers like f16 and f22 have a deeper range of focus (less blur). That seems simple right? But when you understand what is changing about the lens is when you get confused. So if all you need to know is the higher number means more is in focus than feel free to stop reading.
Now that you have read this far, I can tell you what is changing in the lens. Aperture is the IRIS of the lens. The iris is the circle you see in the middle of the lens that lets in a certain amount of light at a given moment. Now for the confusion; the more open the lens is the lower the number is. The lower number means more light. That is why from now on I will say “lower” or “raise” your aperture to reflect how it is changing exposure. “Lower” means to make it darker, so if you have done it properly you will be in the higher F stop values. “Raise”, means to brighten the photo, smaller F stop values.
On a lot of starter lenses the lowest F-stop, or "wide open", is going to be f3.5. But the crispiest photos come in with numbers lower than f2.8 if you are into that kind of thing.
Why is this important to know? Well, like I said it affects the photos exposure. Because depending on how bright the scene is you can open or close the lens off to light. The aperture changes how much light is coming through the lens and is strictly confined to the lens. So if it is bright out, shoot high F stop if it is dark shoot low F stop.
Example 1: Let's say it's 12 O'clock and it's bright as hell. You want to take a photo but your shutter speed is maxed out (oh no!). You see that your F stop is at f5.6. What do you do?
Example 2: Say you have it exposed properly but you want a shallower depth of field. You are at f11 what do you do?
Example 1: Lower your F stop to something like f16.
Example 2: Raise the F stop to something with a more shallow depth of field like f3.5. But make sure to check shutter speed and ISO because they now have to be adjusted as well.
That is it if you want an exercise to help you start. Try shooting on the Aperture Priority. It should be the one labeled "A" on your camera’s mode dial. That way you can see how aperture changes the picture’s appearance outside of exposure. And you can also take a mental note of how the camera adjusts the other settings to accommodate the changing aperture.
Get to shooting!