try the extract plugin and adjusting black/white input sliders
Depends on what it looks like. Anything from a simple Shift Channels based on Luminance and Inverted to more complex channel operations including multiple adjustments such as Levels is possible...
White can be extremely difficult to remove. The best and most effective procedure depends entirely on the shot. Your description of the shot doesn't tell me much and I have no idea what you are trying to do in the comp so some more details would help us to point you in the right direction. It might be as simple as changing the blend mode to Multiply.