I import a video which represents a character, but this video has a black background, whereas my after effect composition has a white background.
How can i do to cut up around the character to put transparent (or white) the black background of my imported video ?
You can remove the black backround using a variety of methods. A few ways you can explore are, Keylight, Luma Key, Transfer Modes, rotoscoping, or if you have CS5, use the roto brush. Type any of those words that are bolded into the community search, and see what comes up!
Yes, but it would have been far easier if the "imported video" had been shot or composited with this keying operations planned ahead of the post production phase.