|Classic: AF chooses wrong point.|
You may have noticed that cameras these days advertise as having more and more focus points '19-point autofocus system' etc, their patchwork of little points covering an increasingly larger portion of the cameras view. You can sometimes see these little points light up when you half-press the shutter button to focus. Devastatingly, by default, unless you've changed it (or if your camera mode is set to Auto) all of these focus points are 'active' and independently searching for something - anything - to focus on, and so when you half-press to focus, the first focus point to latch onto something 'wins'. It could be any of them, focusing anywhere in your frame - and very often it's the wrong point, or at least the wrong part of the right thing. As you'll learn later with controlling your aperture (if you don't know already), part of the joy of some photography is being able to have only a thin slice of stuff in-focus (ie a small depth of field), with everything in front and behind beautifully blurred out. In these situations particularly, focusing on exactly the right part of the person or animal (their closest eye) is critical - somewhere on the body isn't good enough.
|Just using centre AF point.|
Don't be fooled into thinking that by having all of these focus points turned on that your camera is somehow focusing on 'all' of the points - it's not. A lens can physically only focus at one specific distance at a time. By having all the AF points active, it just means you're letting the camera decide which single point to select. So considering as your lens can only focus on one point in the end anyway, it's crucial that you chose exactly which point in your photo this is to be - ie your subject, not the blades of grass in front of it, or the mountains behind it.
|AF point selection button.|
Thankfully, in all camera modes more advanced than 'Auto', you can select which AF points you want the camera to use (ie all of them, just some of them, or only a particular one). You'll either have a dedicated AF Point selection button (most Canon's do), or perhaps it's inside your menu (most Nikons it's under AF -> AF Area Mode) and honestly, for 95% of you photography, you're far better off setting it to ONLY USE THE CENTRE AUTO FOCUS POINT (single-point AF, and set to be the middle point).
Yes, this means you have to point directly at the part of your photo you want to be in focus while you half-press, but as you'll see in the next section, providing you set your Focus Mode to 'Single Shot' / AF-S mode, after it's found that focus, providing you keep your finger half-pressed on the shutter button, you can then re-frame your camera to position your subject wherever you want within your scene (for better composition, such as abiding by 'The Rule of Thirds') before you then press the button all the way down to capture the photo. In this way you can very easily, quickly and accurately select exactly what part of your scene or subject you want in sharp focus, but still place it wherever you want in the shot.