Meter the face, shirt and background -- and make sure it is less than about a 5 stop difference.
I would likely set the exposure between the metered readings for the face and shirt, adjusting by how dark the person is.
If your wanting the background to be totally white (with no detail) while keeping detail in the shirt, place a backlight either shining directly on the background or thru it. The separation of background and shirt will likely require you to keep the light from falling too heavily directly on the shirt. That is, in general, since they are both white if they are lit the same there will be no separation, so the lighting must provide it.
If you have complete control of the lighting, meter the face and set the exposure to -1 or -2 stop compensation. Then while keeping the lighting on the face constant, adjust the lighting on the shirt and background. The shirt will likely be good at about 2 or 3 stops above the face, with the background placed as you like it.
One thing to keep in mind if you don't have complete control of the lighting; the face is usually the most important detail of a person. If exposure compromises must be made, make the compromises in the other areas of the photo and make sure the face is well exposed.
This is how I understand it, but keep in mind that I am a novice and the more experience will likely respond fairly soon.
David
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