On many cameras the focus is achieved by half depressing the picture taking button just before shooting. So a novice will press the button fully down not realising that focus has been done by the camera.
Focusing is telling the camera what part of the image in front of you it is to concentrate on. It is important to tell the camera the light level, colour and depth of field especially if you are not using auto.
If you read about focus in your manual it will explain how you can set the camera to show you the focus points it is using and to move them around.
On more expensive models you can configure the buttons so you can move the focus from the picture taking button to another button dedicated to do just focusing. This is called back button focus and takes a bit of getting used to but gives you more control ultimately. A properly focussed image will show clearly in the view finder. you can then use the magnification button and move around your screen to check all the key points are in focus before taking the picture.
If you have the live view feature you can see that the camera will be looking at to take the picture. If you have a touch screen you can also touch the point where you want the camera to focus.
If you take a picture where the key element is to one side or near the front you can move the camera to focus on that first and then without refocusing move the camera back to the original full on shot. This is an alternative to moving the focus point within the camera.
For example in a Landscape focus on foreground before taking the bigger picture and Wildlife focus on the birds eye or sports the participants eyes.
Your camera will probably have within its setting a number of ways it can focus too, with single spot being the most accurate. Ensure when doing movement shots to use Ai –servo for tracking , this allows the camera to continue to focus, once the first focus has been achieved.
Always make sure you image stabiliser (if available) is turned on the lens , especially if using a long distance lens.