To achieve a white background, it’s not enough to have a white backdrop. You need a backlight. This is a lamp placed behind the subject and adjusted to illuminate just the background. You achieve white only by correctly adjusting the brightness of this lamp. If non-adjustable as to intensity, you must use distance as the basis for the adjustment.
You need to light the background quite brightly but it’s diffucult to achieve a uniform white with a simple lamp. This uniformity can only be obtained using a “broad” source as outputted by a bank of highly diffused lamps.
The rule is get 2-3 stops more light on the background than on the subject, metering and exposing for the subject, of course.
Obviuosly you have to care the subject lighting, too: First the “fill” lamp: draw an imaginary line between subject and camera lens. The line passes through the camera lens and beyond. Your “fill” lamp should be situated as close to this line as possible. Place “fill” to shine on the subject, off to the side only enough to avoid obstructing the cameras view. If behind the camera, position off to the side just far enough to avoid casting camera shadow on subject.
Place “Main” high and off to side, turn off “fill” while adjusting position of “main”. Watch subject’s shadows as you move the “main” for effect. Remember, “main” must be brighter than “fill” therefore closer.
Another way to get white background is use photoshop or other editing programs. You have to mask off your subject and use levels to “bring up” the back drop.
Before long the step by step workflow.