Shallow Depth of Field
There are actually three factors that will affect your ability to reduce your depth-of-field and blur the background in a portrait.
The first is the aperture. The low number apertures (i.e. f/2, f/2.8, etc.) will produce your shortest, shallowest depth-of-field’s. In fact, the lower the number, the shallower the depth-of-field.
The second factor is the lens’ focal length. The longer lenses will have inherently shorter depth-of-fields.
The third and final factor is the camera-to-subject distance. Don’t expect to have a beautifully fuzzy background using a 50mm lens with your subject at 20 feet. Instead, consider using a lens in the range of 85mm to 135mm at a distance of 8-10 feet.
You could actually use any telephoto lens and I’ve seen some shooters using 200-300mm lens on occasion, but lenses in the 85-135mm range are usually considered to be “portrait” lenses and using this combination does provide a few benefits. First of all, instead of being right on top of your subject, the greater distance moves the photographer out of the subject’s personal space making them more relaxed. Then the gentle compression of the short telephoto lens usually has a flattering effect on the subject’s features. Combine that with the aforementioned shallow depth-of-field of a smaller f/number and you have a lot of the makings for a wonderful portrait!