With the default camera settings, the camera will decide for you what to focus on. The results are not always what you would want. Of course, it would be much better for you to choose for yourself what you'd like to focus on!
First of all, there is a little switch on your lens that needs to be switched from MF (manual focus) to AF (auto focus).
Then take a look inside the viewing area of your camera and you'll see a set of focus points that looks more or less like this:
You're going to set one of those points as you "go-to" point. I almost always have my set as the center point, just because that way I always know where it is and it's totally a habit for me to go right to it whenever I need to. I suggest you start out with the center point chosen, and then after you get comfortable, you can decide if you'd ever like to change it.
On a Canon camera, there should be a little button that looks somewhat like this:
If you push that button you can scroll through all the focus points to choose the one you want. You will be able to see the little squares light up red in your viewfinder so you know which one to select. Or, some camera models (like mine) will show your selection in the top screen next to your shutter button.
On a Nikon camera, once you're ready to shoot (touch the shutter button to get the camera in "shooting mode") you can just look through your viewfinder and use your arrow buttons to select your focus point.
Now you need to choose a focusing mode. On a Canon, look for a button (or a menu selection) called AF Drives. Then you can select one of the following (on Nikon look for the AF button):
One Shot (called AF-S for Nikon) is good for stationary subjects such as inanimate objects or people who are not moving (adults/older children who are posed). With this setting, you look through the viewfinder and place the center square directly over the subject's eye and press the shutter button half-way down to lock the focus. Now as long as you keep your shutter button half-way down, the focus will stay locked on the subject's eye and you can move the camera to recompose the shot the way you'd like (you probably don't want their eye right in the middle of the frame). Press the button all the way down when you're ready to take the picture.
The other focusing mode is called AI Servo (or AF-C for Nikon). This is used for moving subjects (sports, fidgety kids, interaction/candid photos). With this setting, you place the center square directly over the subject's eye and push half-way down to focus and do not recompose. As long as you have the button half-way down, the camera will stay focused on whatever is in that center square, even if it's moving. Press the button all the way down when you're ready to take the picture.
If you don't want your subject directly in the middle of the picture, you can always crop it to your liking later.
Or if you'd like to avoid having to crop your images, this is a situation where you might want to choose a focus point other than the one in the center.
There you have it! Now you know how to focus like a pro! Here are some things to keep in mind...
Some lenses are faster-focusing than others so they'll perform better when using AI Servo (AF-C).
A person's eye is the most important part of the picture, so unless you're going for a creative, selective focus shot, always focus on your subject's eye, whichever one is closer to you. If the person is far away, just focusing on their body is usually fine.
There is a third focusing mode (AI Focus/AF-A) in which the camera goes back and forth between the other two modes as it determines is appropriate. But the camera is not always correct, so I prefer never to use this setting.
When shooting at wide-open apertures with a very shallow depth of field, accurate focusing is very important! With practice using these techniques, you'll do great!
