As important as the exposure of pictures (Link) is the right focus and you can as well choose between an automatic mode (Autofocus (AF)) and a manual mode (Manual Focus (MF)). You can configure the mode with a switch on the lens. For feedback about the focus you can either use the viewfinder or the rear LCD.
Hint: To get the right impression about the focus with the viewfinder don’t forget to adjust the diopter of your camera.
Autofocus
To calculate the right focal distance cameras use either phase detection (viewfinder) or contrast detection (rear LCD). With the area mode, like Single point, Dynamic area or Auto area, you can choose the preferred focusing strategy.
Hint: If your camera doesn’t want to take the picture it’s possible that your camera is set to focus priority and this means it just takes pictures if it believes the scene is in focus. If that happens you can either set it to release priority or switch to the manual focus.
Manual Focus
If the Autofocus isn’t focusing as expected or makes the procedure of taking photos too slow you should switch to the manual focus. Use the wheel on the lens to set the preferred focal distance.
Hint: Normally the focal distance is quite obvious, except when you are doing a landscape shot. There you have to work with the hyperfocal distance.