In photography a Shallow Depth of field is achieved by using a wide open Aperture (Low F-Stop #)
Hopefully the result will be good focus on your main subject, with a nicely blurred background and/or foreground. This photo of Happy is a good example of that.
You can also go for a different result by focusing on a different area. In this second example, I focused on the Tennis Ball leaving Happy outside the cameras depth of field and therefore out of focus. In this case the camera was on auto focus and the mis-focus was accidental. However, I kind of like the result.