@watdapakisdis
Let me try to cook it down to an easy example using your above pic:
1. delete all cameras - usually there appears a default one. This is just to make it a proper habit
2. Move your view in the viewport to point the way you want the final picture and go to 'Create..' menu - create new camera with option 'copy viewport setings'
3. change your view in the viewport to use that camera now
4. select camera in scene and change to 'Parameters' tab
5. in that camera one option is called 'Camera' - select that
6. Click 'on' on Depth of field in here
what do those other options do is probably what you want to know most - so here is goes:
Imagine the camera has an angle when it points to something. The '
Frame width' option makes this angle wider or smaller thus making the viewed frame wider or smaller. The frame is important as that is actually the focus of your camera.
'
Focal Distance' is the point where the actual focus happens. Everything in front or behind that area will be blurred. I personally suggest to not use 'point at...' on the camera as the more subtle results can be achieved by manually using the options. Combined with the option '
Focal Length' you can create a kind of box where the focus is high thus the elements appear sharp where everything outside of the box will be blurred. Finally there is 'F/Stop'. This option will determine how blury things will be outside the actual focus area. The higher this value the more things become blurred.
With some exercise you can create fantastic small focus areas around the eyes for example and create renders that look very professional. To see where the actual focus plane/box appears you should change your viewport view back to 'Perspective view' and look how and where the box and the line of sight move when you change parameters. Optionally you can also drag the control elements directly in the viewport.
P.S. DoF as an effect on the camera is just one thing to make it 'pop'. Lights are actually the second most important thing to make DoF work at all. The more elements are in your scene the more you have to take light and F/Stop into proper consideration whereas in portraits its a lot easier to get good effects.