You'll have to experiment to get the results you want.
Focal Length, Focus Distance and Aperture are your 3 variables for controlling depth of field.
Sometimes, you can focus just ahead of your subject (so that it's barely in the area of acceptable sharpness) to help out.
If you just can't get the results needed any other way, you can also use an editor to blur the background more selectively. If you search through the Editors Forum, you'll find some posts on this subject, like this one: