my guess it that you want to get min DOF from your lens. yes?
some factors that affect
1. focal lenght, the greater the focal length the lesser the DOF
2. aperture, the greater the aperture number (eg. f22) the greater the DOF
3. camera to subject distance, the lesser the distance the lesser the DOF
and lastly to increase the background blur from your subject, the greater the subject to background distance the greater the background blur.
i hope that this is not too technical to understand