I love the mountains in the background it sure gives some dimension to the photo. Where was this taken?
As for the question...
The wider open the lens (the smaller the f-number) the shallower the depth of field (DOF). Conversely, the smaller the aperture (the larger the f-number) the greater the DOF
As Jan said, set your F-stop or F-number the lowest number possible. This will depend either on your camera or your lens. So you are looking at around F1.8 to F2.5
hugs
Emilie
Looking forward to seeing some nice DOF